The original 2004 Gmail iteration of the now-ubiquitous modern status bar (here presenting undo send) was internally nicknamed a butter bar because… well, just look at it:

(I believe at least Google today calls this a snackbar.)

The UI pop-up element hosting Google Talk inside Gmail – the very same thing that’s more commonly called a “toast” these days – was originally termed a mole:

The column view in NeXTSTEP was called a browser, but a few years later someone put together a different kind of a browser on that very same machine, and the original term has been sunset – after NeXTSTEP became Mac OS, the view was renamed to “column view”:

These three are off the top of my head. Please send in more!