On the level editor, while using all 10 slots, a storyboard will appear. The following messages are...
1: I think that's enough, don't you?
2: You've used all ten hotkeys!
3: Could you simplify your level design, rather than adding more tiles?
4: You can make lots of interesting things with just two or three tiles. This is more than enough!
5: By the way, if you accidentally clicked the wrong tile, just reload the editor to get rid of it.
6: No but seriously, you get nine tiles plus the eraser, and that's it.
7: Clicking more won't make me change my mind.
8: You're just clicking to see what else I say, aren't you?
10: Did you know? This game now has a wiki! Check it out at run.wikia.com.
11: The wiki editors have been doing a great job uncovering the secrets of this game, but they could always use more help.
12: Ok, that's all. Get back to building that level!
The last message will repeat infinitely.