Win7x64. Was able to reproduce this twice.
I was doing some SQL script comparisons trying to find any differences between certain stored procs to track down a bug.
Block copied the target proc from the source file and, to simplify things I created a new file (file | new) of an SQL server type and pasted the code there. Then I...
- Ctrl- = to bring up the diff and specified file diff from the dialog
- Selected the unsaved buffer I just created as file 1
- Selected the target file that has another copy of the proc as file 2 and did the diff (and there was no difference, grumble grumble but that's not pertinent here...

)
I closed the diff window and went to copy and paste another proc into the buffer and SE won't let me because it says the "file is read only" (this is the buffer I just created) - Sure enough the state display on the lower right says RO.
I tried reproducing with the steps above except this time I minimized the diff and went back to SE first, then (using the app bar) brought the diff window back up and closed it.
THIS time, not only was the newly created buffer in RO state but all the diff highlighting/text was still in the SE windows. Rerunning the diff I can sometimes affect the highlight text but I can't seem to unset the RO state of the buffer.