I'm aware that the dungeon exit and the subdungeon entrance have different graphics, but the evidence suggests that that's not enough to prevent the mistake. (Also, the dungeon exit and the subdungeon exit, which is harmless, look identical.) I guess I'll make an effort to double click on subdungeon ladders, but if there should be an update to the game I'd really appreciate press-space-to-kill-yourself being disabled or disablable.
There's no confirmation dialog to leave the dungeon by pressing space or by clicking the "retire" button that presents itself when you're standing on the ladder, but there is one to leave by selecting "surrender" in the menu. This seems like a funny choice, since that's the suicide method that's hardest to reach by accident. If I were to speculate wildly, I'd say that...
1. Double click to kill yourself was disabled because someone discovered that it's easy to double click on a tile when you meant to single click.
2. Space to kill yourself wasn't because it's hard to hit the space bar inadvertently.
3. "Surrender" has a confirmation dialog while the other two don't because "Surrender" destroys your locker item even if you had a path to the exit. Frankly, I think it'd be a better design to let you keep the locker item when you "Surrender" and only require relockering if you consumed or converted the item. :/