Currently if you select and copy text you get all the lines in all versions of the document, whereas presumably you only wanted the lines that are for the currently visible document(s).
This can probably be fixed using some kind of hook into ace's copying machinery..it will need to be a monkey-patch style hack like the rest of the specialised stuff in the revision history viewer.
On a related note, it might be nice to make it easy to revert to an old version without explicitly having to copy and paste.
This request was originally posted in Google+.