I have encountered a problem where UPX refuses to compress exes compiled with the latest GCC.  This did not happen with the 3.xx series.  The message that UPX gives is 'unsupported COFF header'.  Perhaps there could be an option to specify useage of older headers?  Or could someone fix UPX to recognize the new format?

I've done some more testing, and it turns out that the problem only occurs if there are external libraries linked in.

UPX old versions up to 1.24 failed to correctly compress
DJGPP executables. They run OK, but after decompressing
to non-compressed state crashed. As result when 1.25 refused
to compress DJGPP executables, which this situation were
detected. The problem was fixed beginning from UPX version 1.93.
Now we have 3.0.

So I guess this bug should be closed as fixed