Old Composers and rm after seven
In Never use rm afte seven, Ruth writes about a backup that wasn't and a way to take advantage of caches to get some files back.
I had a similar issue once where I'd created an Xcode project with the wrong settings, then another with a very similar name, and then trashed the wrong one. After I was done with the new project (which was still open in Xcode and through the magic of Aliases now pointed to the copy in the trash), I emptied the trash and suddenly most of my files were red. I was able to restore most of the files by simply saving the still-open files, but a few central source files died that day :-(