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Very infrequently, a very strange thing starts happening, seemingly without cause. At first, it seems as though my keyboard has broken, but replugging has no effect, and bringing up OS X's keyboard viewer DOES display keypresses.

However, nothing is inputted to the system or any text fields. In fact, clicking with the mouse on individual keys in the keyboard viewer also has no effect.

Logging out and logging back in fixes whatever the problem was. How would I go about troubleshooting this, should it happen again?

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Thanks for the accept -- what was the issue? – Daniel Beck May 11 '11 at 17:01
@Daniel No idea--it's not reliably reproducible. I typically run with a ton of background stuff, though, so one of those is likely the issue. Your answer is comprehensive, and it's unlikely that there exists a "magic bullet" of the nature I was hoping for. – NReilingh May 11 '11 at 17:13
Too bad. Good luck though. Maybe reinstall the system and keep only the user data? Don't know if that's feasible for you... – Daniel Beck May 11 '11 at 17:20
Another user seems to have a similar issue. Maybe you two could compare your third party software in use to see if you both use some software that might be the cause? – Daniel Beck May 21 '11 at 6:07
up vote 1 down vote accepted
  • Open /Applications/Utilities/ and look for relevant messages.
  • Log in to your Mac using a remote desktop software (e.g. VNC) and see if key presses work from there
  • Run ps aux in Terminal (/Applications/Utilities/ and look for suspicious processes.
  • Disable all processes that run in the background, including:
    • processes started when you log in (Accounts preference pane),
    • started from third party System Preferences panes (~/Library/PreferencePanes etc.),
    • launchd property lists in */Library/LaunchAgents and */Library/LaunchDaemons
  • Uninstall all third party kernel extensions from /System/Library/Extensions
  • Remove all input managers from */Library/InputManagers
  • Log in to another user and check to see if that account is also affected (including the Login Window itself)
  • Run sudo fs_usage -f filesys in Terminal to see whether some files are accessed very frequently
  • Change the keyboard layout in Language & Text preference pane
  • Toggle support for assistive devices in Universal Access preference pane
  • Check if some options in Universal Access preference pane are enabled, e.g. slow keys
  • Use a virtual keyboard or AppleScript to emulate key presses and check if they get through.
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It just happened again, and the keyboard viewer behaved exactly same as the OP described. It will be interesting to collaborate and narrow down to the app that might be causing this issue, but I don't see a mediawiki like talk option here, and I don't think adding multiple answers to this question would be appropriate (and btw., the limitations on comments don't make it conducive for this purpose).

In any case, the reason I am posting this answer is because I found a better workaround than the OP (which was to logout and log back in). When I simply put the display to sleep using the Ctrl+Shift+Eject, I was able to enter password to unlock the screen, and voila... the keyboard was working again. This is a huge relief for me because I am actually in the middle of something serious and restarting my session would take a lot of effort.

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Very interesting; thanks for the feedback. If I get this again, I'll definitely try that out. – NReilingh May 27 '11 at 3:03

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