bash on OS X
Funny thing happened... I had a directory with 2 dmp files. One was compressed using gzip, and one was un-compressed. When I typed gzip -d and pressed tab, it listed the only compressed file which had a name ending in gz. No matter how many times I pressed tab, it didn't pick the other file which I know exists in that directory. So obviously, me being me, typed gzip without -d on the shell and pressed tab... and voila, it listed the uncompressed file and didn't list the other file which was already compressed. I've been working with bash for a long time, and haven't noticed such a feature before... I was pleasently surprised to find it.