In a homepage with: <SCRIPT LANGUAGE="JavaScript" SRC="somefile.js"></SCRIPT> in the HEAD tag, if you visit the page, change the somefile.js file on the web server without closing the browser or clearing memory cache, and then reload or revisit the site the somefile.js file will be loaded "on top of" the previous somefile.js instead of overwriting it. For example, if somefile.js is 10KB but then a developer optomizes it and makes it 5KB, then reloads the page that references this somefile.js, errors may occur. If you look at the .js source you will find that the file is STILL 10KB because the new 5KB file has been simply over-laid the 10KB file, still leaving the end 5KB of the old .js file at the end. This would not usually be a problem in a normal environment, but in a development environment this can be troublesome. Clearing memory cache and using the Debug-->Flush Memory feature forces Mozilla to reload the .js file and this error can be circumvented.

Browser, not engine. Reassigning to Networking for further triage -

this is an already-filed cache bug but I can't find it....

Yup. *** This bug has been marked as a duplicate of 79983 ***