Make sure the chainsaw chain
is correctly tensioned. A slack chain may jump off the guide bar, injure you and damage the chainsaw. Overly tight chain tension can cause premature wear of the guide bar. A properly tensioned chain should not hang under the guide bar. The chain tension is correct when the chain is in contact with the underside of the guide bar and you still can pull it around easily by hand.
Always let the chain cool before adjusting the chain tension. As it cools off, the chain will shrink.