I didn't really do much alpha testing for Magicville - only two people other than me saw it before Beta. At Beta I had I think seven people plus me testing and they found a lot of stuff - the game was played from start to finish twice over by one person, plus the other testers trying to break it, before I was happy to let it go. It was in beta for about six weeks.
When I released Magicville it had a doozy of a bug in it, caused by third party code.
The code I used attempted to set permissions for the "Users" group... the code referenced this group by name, not its constant. This turned out to be very, very bad, as the group isn't called "Users" on any non-English OS, causing the whole game to fail.
How did I miss that? Well, I'm English and have an English OS! I didn't see it coming in a million years but come, it did. I got a revised build out within a day but it still took a few days to filter through to market. My conversion rate took a nasty hit (the bug-free version's conversion rate was 3x that of the first release). I don't like to think about how much money it cost me in lost sales.
How will I stop it happening again? It probably won't, now that I know! But even so, I have the German Edition of WinXP running on my Mac via VMWare.
When people find post-release bugs I try to be apologetic and positive in getting it quickly fixed. People usually understand!