I get the following error on OS:X 10.11.2 Beta (15C31F)
alexander@rincewind:~$ upx
Illegal instruction: 4
The package is installed via homebrew:
alexander@rincewind:~$ brew info upx
upx: stable 3.91 (bottled), HEAD
Compress/expand executable files
http://upx.sourceforge.net
/usr/local/Cellar/upx/3.91_1 (8 files, 1.8M) *
Poured from bottle
From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/upx.rb
==> Dependencies
Build: p7zip ✘
Required: ucl ✔
If I build it from source, it works with no poblems, so it must be a problem with the binaries you distribute.