How to stop the chain from coming off the bar

There are three main reasons for a chain to come off:

1. Incorrect bar and/or chain for the saw; check to be sure that you are using the proper chain pitch, gauge and number of drive links to match the guide bar.
2. Chain not properly tensioned; It is normal for a new chain to stretch and require frequent adjustment. A properly tensioned chain when cold is tight enough to not sag below the guide bar, but loose enough that you can move it by hand (CAUTION! Only do this with gloves on!) with some light resistance. A hot chain will have slightly less tension. If you need to adjust it while hot, allow it to sag below the guide bar about the width of a medium screwdriver tip. The drive links should never be completely out of the guide bar channel.
3. Trying to cut brush; the small branches may become lodged between the chain and sprocket, kicking the chain off track. Use a hedge trimmer or brush cutter instead for this application.

How to tension a chainsaw chain?