just follow these steps, and nothing will go wrong this time.
Player -> collision -> any side of wall -> Physical response ->
there are 4 textboxes with a value 1.
change the third one to 0.
then click "ADD", "immideate action" and everything will be just fine.
what am I saying???
you can also check out makslane's turtorials for collision.
go on help -> game stuff -> gravity and follow the turtorial.
hope that settle s it.