This has becoming a problem for me recently. Sometimes when I alt+tab, the game quickly closes without any sort or warning or error. This isn't a huge problem, and it has been addressed several times as likely an "outdated driver" problem.
The main problem is when I try to get back online. The game thinks I'm still online and my Spiral Knights crashes whenever I log into the same account. If I'm running with my friends, they tell me through Skype that my character was standing still in the dungeon I disconnected from and I was on their friend list. Every time this happens, I have to wait out the inactivity timer and I have to restart the whole dungeon. Possibly worse, any party I'm with has to either kick me or all go solo-losing most of the party member's drops.
Does anyone know a way to prevent this or a way to let the game know you're offline?
Well, it makes sense, the game crashes so obviously the session isn't terminated properly.
You'll have to wait 15 minutes so it threats you as timed out.