This error was bugging me for about 2 days, so I though I would post this to a) help me remember to code properly and b) help someone else solve it in the unlikely situation of it happening elsewhere.
Basically, in IE8 and IE8 only, I was getting a helpful error message that said: line 1421 in jquery.js: Failed. Line 1421 was because I had uncompressed jQuery to try and find the problem.
The only other example of this error I could find (also found by Kenneth) was this instance: When jQuery returns “failed” in IE – and how it’s probably resolved which while it helped a bit, we weren’t using any live events.
The absurdly easy solution following hours of troubleshooting, was upgrade both jquery.js and jquery.forms.js to the latest versions. Bosh. Have that IE8.