To summarize this issue:
First Rob added nice new evilness to MDW so that sometimes Directory.deleteFile would fail even if Lucene did not have that file open, simulating a virus checker temporarily holding the file open. (Previously this operation would always succeed).
But this new evilness uncovered a nasty corruption case in Lucene, whereby 1) an unclean shutdown of a previous IW left some "future" segment files in the index, e.g. _5.pos, 2) the new IW starts up and identifies this file as not being referenced and immediately tries to delete it, but 3) the virus checker prevents _5.pos being deleted on init. Normally this is "ok": IW records that this file needs deleting but failed last time and so it periodically retries.
The problem is, when the IW goes and flushes a few segments, it may now in fact overwrite _5.pos with a "real" one, which may succeed (if virus checker is done with that file), and then later when IW retries its deletes, it removes _5.pos, corrupting the index.
I don't know of any actual user cases showing this corruption case ... but it's quite insidious ...