Suddenly I noticed that my beloved TextExpander just stops working/expanding for no apparent reason. Restarting the app didn't work, but what does work is restarting Firefox.
I guess there's some conflict between Firefox version 3.6.x and the TextExpander engine. So the quick fix seems to be to just restart Firefox when this happens.
Evidently, this isn't fixed in TE3 - so no need to upgrade with hope of this getting fixed (at least not of this writing).
Cheers!
UPDATE January 4, 2011:
It has been pointed out in the comments that closing Chrome can also fix the problem - thanks folks!

By Jean MacDonald June 2, 2010 - 6:38 pm
Hey, Jean from SmileOnMyMac here. There is a bug in Firefox and Chrome that enables secure input and then fails to disable when it’s no longer needed. This affects all apps that use keylogging, TextExpander included. The bug has been filed and hopefully will be fixed soon.
By Matt June 3, 2010 - 7:29 am
Awesome, thanks for the update Jean!
By Kohl Gill November 26, 2010 - 2:30 pm
Is there bug fix link for Chrome?
By Donald December 31, 2010 - 8:51 am
I found that simply closing Chrome fixed my problems.
By Brett January 4, 2011 - 1:07 pm
+1 on quitting Chrome. Thanks for that, Donald.
By Brock Boland January 5, 2011 - 3:43 pm
Sweet rutabaga, thank you Jean! TextExpander has been crapping out on me every now and then for weeks and it was driving me crazy. I hope that bug is fixed in Chrome soon.
By Thomas January 26, 2011 - 7:36 pm
And I just thought it might be a good idea to use Chrome for fast browsing on my old MacBook.
I haven’t got that issue ever with Firefox, though. Gladly.
Thanks for the info! I’m just wondering if it’s Chrome or Apple who are to blame.
Thomas
By Thomas February 2, 2011 - 4:15 pm
Add 1Password (old news) and ControllerMate to the list.
I wasted 10 minutes trying to find out why Alt-ShiftLock did not print a file… best guess was my new Logitech mouse.
Then: mh, what am I using to find some help? Chrome. q.e.d.
By John Wiegley March 3, 2011 - 1:23 pm
I’m in this same situation, but neither Firefox nor 1Password is running; and restarting them doesn’t have any effect. How else does one disable secure input?
By Guy April 3, 2012 - 12:59 pm
The problem in Chrome is caused (at least in part) by a bug in the 1Password extension, if you have that installed. It’s triggered when instead of unlocking 1P (either the bar or the box) you just close it without unlocking.
If this is the case for you, complain directly to 1P
By James March 26, 2013 - 6:59 am
Uninstalled the 1Password extension on Chrome and it is working again. Thanks!