I just found a bug in 7z rebuilding. I downloaded two .7z files from the newsgroups and did a rebuild using them as a source. The two source files contained 8 files total (5 in one, 3 in the other). During rebuild 4 of the 8 were matches. Instead of removing the files from the archives I was left with the same two archives (unmodified) and a new empty (32 bytes) .7z archive.

Before:
1,030,401 Pete Sampras Tennis 96.7z
619,964 Pete Sampras Tennis.7z

After:
1,030,401 Pete Sampras Tennis 96.7z
619,964 Pete Sampras Tennis.7z
32 Pete.7z