Note that bad features, poor interface design, and problems with third-party apps, etc., are listed separately here.

Following the release of Sierra 10.12.6, this list has been superseded by the new list for 10.12.6. This list is no longer maintained: please refer to that last for the latest information.

If you are looking for a list of bugs in previous versions of Sierra, that for 10.12.4 is here, for 10.12.3 is here, for 10.12.2 is here, for 10.12.1 is here and for 10.12 is here. Note, though, that those are no longer maintained.

Touch Bar firmware update – Kernel panics and infinite boot loops on some MacBook Pro models with Touch Bar

There are reports that some users with the new MacBook Pro models with a Touch Bar are discovering their Macs are suffering kernel panics, or even getting trapped into an infinite boot loop, after updating to 10.12.4 through the App Store. This seems less likely when updated using the downloadable installer from here.

Suggestions for dealing with this are to restart into Recovery mode, open Disk Utility, and run First Aid on the startup disk, but some Macs have had to be set up from a fresh install of Sierra. This may relate to a failure to complete the firmware update to the Touch Bar. Full details are on jamf Nation.

Display sleep – multiple display setups may not wake properly after sleep

A few users, mainly with iMac 5K models, are reporting that allowing display sleep with one or more external displays connected causes problems when waking. Although the external displays wake normally, the iMac display does not. So far the only way to restore a normal image on the internal display is to restart. This is new with 10.12.4.

The workaround is to disable display sleep if affected, and rely on a screensaver which darkens much or all of the display.

(Thanks to Richard Petersen for reporting this.)

Time Machine and other background activities – irregular and unreliable after long periods of running

The long-running bug affecting undocumented systems in macOS which schedule and dispatch background activities, most notably including Time Machine backups, has not been fixed in 10.12.4. Macs which are left running continuously for more than about 7 days are liably to develop this. The most obvious symptom is that scheduled backups stop, becoming very irregular. Full details are here. If you’re unsure whether backup irregularity is the result of this bug, use DispatchView (in Downloads) to check.

The only solution is to restart the affected Mac.

This bug was reported to Apple in February 2017. I had originally thought that it might have been fixed in 10.12.4, but it remains.

Energy Saver pane – inconsistencies in controls, and misbehaviour on some models

The features available in the Energy Saver pane of System Preferences are inconsistent between Macs, and incomplete, for example not providing a separate control slider for system sleep on some hardware. Further details are here.

A further bug affects some models, including the iMac17,1: irrespective of the setting to put hard disks to sleep (ticked or not), system sleep invariably puts hard disks to sleep. The only way to prevent hard disks from being put to sleep is to prevent system sleep altogether.

These are long-standing issues which date back to El Capitan.

Safari – lost duplex printing option

New with 10.12.4 is the loss of duplex printing options for certain Canon and Epson printers. This is also true of Safari Technology Preview.

The workaround is to print (or save) to PDF and print that using an app which offers duplex mode.

Thanks to Henry for reporting this below.

USB Audio – various problems with output

There have been many reports of various defects with USB audio output, mainly with headsets, but a few to speakers. These include broken, choppy and distorted audio. Some report that Apple is aware, and there are rumours that these are fixed in 10.12.5 betas. No other fix or workaround has been discovered.

Thanks to mcgroarty and AppleInsider for reporting this.

Preview – loss of all items from Services menu

Preview, and other apps which rely on PDFKit, no longer support any items in the Services menu. Those which had previously been accessible are no longer there.

There does not appear to be any workaround.

Thanks to derdennis for reporting this below.

Console – no access to full historic log data

The Console utility does not provide full access to log entries already gathered before the app is opened. This has been the case since Sierra 10.12.

Workaround: use the log show command in Terminal, or export the whole log to a .logarchive file, then browse that using Console. You may find it preferable to use my free Consolation, available from the Downloads item above, which provides sophisticated facilities for browsing log entries which have already been made.

log (command) – bug in collect option

The log collect --size option still does not restrict the size of log entries output – it is functionless and there is no workaround.

Finder – incorrect column width

This can occur when using Finder windows which are set to column view. When switching folder in the view, the rightmost column being displayed has excessive width, filling the Finder window, its divider being placed incorrectly at the right edge of that window.

This long-standing but intermittent bug dates back to Mavericks if not earlier, and I have whinged about it here and here. It was also present in every version of El Capitan. The only workaround is to select a different folder, then to select the correct folder again.

Finder – inconsistencies and other bugs in List views

There are several bugs in the Finder’s handling of maximize (zoom), which are most prominent, and perhaps largely confined to, List views. The most obvious, reported here, is that maximising List views often doesn’t result in a window which is deep enough to contain all the items in the view, even though there is ample space to do so.

Although these don’t have major impact, and can be worked around, they are messy and inconsistent.

Finder and file system (HFS+) – strange behaviours with ~/Library/Mobile Documents

The ‘folder’ ~/Library/Mobile Documents cannot be opened in the Finder, but redirects to iCloudDrive. The odd behaviours associated with this are detailed here, and are not bugs but ‘features’. The workaround is to access them via Terminal, as detailed in that article.

Apple Magic Keyboard – duplicated letters

Sometimes, when typing normally, letters are incorrectly duplicated although if the key had been held pressed for too long, that would not have resulted in keystroke repeat, but would have popped up the accented character picker. This has been a sporadic problem since El Capitan and persists in Sierra, although it now appears quite infrequent.

I suspect the workaround is to use a wired keyboard.

Bluetooth – spontaneous disconnections and others

Although much improved from El Capitan, there are still problems which may be attributable to bugs in Bluetooth drivers. These include spontaneous disconnection of connected devices such as the Magic Trackpad 2. These are generally infrequent, and now seem innocuous.

OS X internals – clicking on a window causes it to jump

When switching between apps by clicking on a window, sometimes the selected app comes to the front and its window jumps to a new position on screen. This long-standing bug, which persisted through versions of El Capitan, now seems to occur less frequently, but does still happen at times.

The workaround is to switch to that other app by clicking on its icon in the Dock.

security (command) – show-keychain-info writes its output to stderr

The security show-keychain-info command and verb do not return their results to standard output, but to standard error. This may apply to other verbs (not tested). Further details are here.

Workaround: collect its output from the standard error stream when calling from other code.

Corrupt log entries – older apps/tools make corrupt entries in the Sierra 10.12.4 log

Some apps and command tools built using versions of Xcode prior to 8.3.2 and run under Sierra 10.12.4 make corrupt and unusable entries in Sierra’s unified log. This applies particularly to code written in Swift which write integer values to the log. Further details are here.

The only solution is for the developer to build a new release using Xcode 8.3.2 or later; no code change is necessary.

Previous bugs believed to have been fixed in 10.12.4

Random jumping of insertion point with trackpads – this seems finally to have stopped.
System Integrity Protection (SIP) – a zero-day vulnerability in El Capitan’s and Sierra’s SIP which could be locally exploited has finally been fixed.
About This Mac / Storage should now complete, although it can take some minutes to appear fully.
Console now makes it easier (possible) to filter log entries effectively, but still only provides easy access to fresh entries.
In log show, the --start and --end options now appear to work correctly.
Many bugs and issues in Preview and other handling of PDFs have been fixed.

Please add your own experiences and I will incorporate as necessary…

(Updated 17 May 2017)