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Following the release of Sierra 10.12.6, this list has been superseded by the new list for 10.12.6. This list is no longer maintained: please refer to that last for the latest information.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 Note, though, that those are no longer maintained.

Contacts – unexpected quit from Print dialog

Open the Print dialog in Contacts, and Show Details. With the uppermost popup menu reading Contacts (for the app-specific print settings), select the Style of Pocket Address Book. Almost any control which you try to adjust after that will result in an unexpected quit: for example, editing the Attributes, or changing the Flip Style.

There is no workaround: printing from Contacts is at least partly broken for now.

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 was new with 10.12.4, and persists in 10.12.5.

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 still not been fixed in 10.12.5. 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.

The only solution is to restart the affected Mac.

This bug was reported to Apple in February 2017. It has remained unfixed in 10.12.4 and 10.12.5 (see reports in comments below).

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 with a Fusion Drive: 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

A new bug introduced 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, and persists in 10.12.5.

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.)

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.)

App Store – app has lost its progress bar

When downloading some updates, the App Store app may show no progress indicators at all. The only indicator will be seen when viewing the app being updated in the Finder. This is a new bug with version 2.2.1 of the App Store app included in the 10.12.5 update.

During longer downloads, a progress indicator may be shown as previously.

The only workaround is to view the app’s icon in the Finder, which does have a progress circle shown with it.

osascript – open location fails in AppleScript

When entered into the command line,
osascript -e 'open location "http://www.python.org/"'
fails with
execution error: "http://www.python.org/" doesn’t understand the “open location” message. (-1708)
although that code works fine in the Script Editor. This bug appears to be new to 10.12.5.

The workaround is to aim the command at Safari:
osascript -e 'tell application "Safari" to open location "http://www.python.org/"'

(Thanks to defjaf 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 – unresponsive to drag, copy-paste

Sporadically, normal operations in the Finder can stop working. For example, drag-selecting a group of files will work, but dragging those selected files to move them or to open them in an app will not work properly: the files will stay put. Similarly, copying and pasting a selected folder may fail. This occurs infrequently, and appears to be new to 10.12.5.

Workaround: deselect the selected item(s), and try again. It almost invariably works properly on the second attempt. However, the items have to be de-selected and selected a second time.

(Thanks to Dom, in comment below, for confirming that it wasn’t just me.)

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.

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, or perhaps in scheduling background activities. 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 far less frequently, but does still happen at times, even in 10.12.5. It appears most frequent in Apple’s Messages app.

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

Previous bugs believed to have been fixed in 10.12.5

Audio stuttering and distortion in some USB headphones and speakers.
~/Library/Mobile Documents now cannot be opened at all in the Finder, although it can in Terminal.

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

(Updated 21 July 2017)