The first thing to do is check with the author of the game in which you discovered the bug and ask them to investigate. It may be a bug in their code; and even if it isn't, they are the people best placed to report it.
If the bug didn't occur within a game, or you are the game author, you can report it on the Bug Reports forum. Please fill out the entire template as comprehensively as possible when you report a bug; the likelihood of the bug being fixed is much higher when detailed information is provided. In particular, instructions that clearly explain how to reproduce the bug are essential.
Game authors, please note: Many apparent BYOND bugs are actually code problems, so check through your code thoroughly before reporting a bug. If you're not sure whether the problem is with your game or with BYOND, try asking on the Developer forum first.