Summary: If you place signs too fast, the game will crash, the reason: Unknown
UPDATE 28/02/13 4:45AM---------
How to Reproduce it:
1.- Place many signs as fast as you can (Creative mode recommended)
2.- I tested on my iPod Touch 5th and it crashes when you´re placing the 20th-35th.
In the iPod Touch 4th it crashes between the 5th-15th sign you placed.
3.-It doesnt matter if is written or not.
it happens also when you place a lot of signs even if you do it every 10 seconds.