I recently had a problem with some sticky modifier after pressing a button on my mouse with keystroke assignment (even predefined ones like Back and Forward). After some troubleshooting and backtracking, I found that the issue is having Synergy running. After I disabled Synergy, everything works normally. For those who don’t know what’s Synergy, “Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems, each with its own display, without special hardware. It’s intended for users with multiple computers on their desk since each system uses its own monitor(s).” (quoted from http://synergy2.sourceforge.net/).