*NOTE: Do not post confidential information in this bug report.*

What version are you running?
1.5 beta 2, dev builds 2/16 (issue also seen in 2/10)

What's the URL of the page containing the problem?
Issue can be seen on: 
http://reviews.reviewboard.org/r/1410/diff/#index_header

What steps will reproduce the problem?
> On above link, click on "124 lines hidden [Expand]"

What is the expected output? What do you see instead?
> Expect to see lines of code expanded. E.g. on above link, expect last 
hidden 124 lines to be expanded

Instead, the "Loading" icon goes away, and the screen remains as is.

What operating system are you using? What browser?
Verified on XP and Windows 7. Verified on IE 7, IE 8, and Firefox.