When posting a new question on many Stack Exchange websites (such as I am doing now) the preview doesn't work. As I type my question, the preview stays blank. The tags also don't come up with suggestions, I have to type blind and hope I get the right tag.
I can also go back and edit my post, and the preview and tag suggestions work then (so this is normally how I tag my posts - make up a tag, post, edit, fix tags.)
Looking in the JS error console I see this when loading the page for a new post:
Timestamp: 01/04/13 09:33:02 Error: NS_ERROR_FAILURE: Failure Source File: http://cdn.sstatic.net/js/full-c.js?v=ff3a7ddcc8dd Line: 109
I have cleared my cache so I'm definitely getting the latest version of the file, and I have tried it on another machine and get the same issue, however I am only able to test with Firefox (19.0.2) under Linux on both machines. This issue has been affecting me for a few months though, so it's nothing new.
Oddly enough the preview is working fine here, on Stack Overflow Meta, but it does not work on the normal Stack Overflow, Super User, or any of the other handful of non-meta Stack Exchange sites I tried.