Stihl Chainsaw Keeps Going Slack , Loose & Throwing Chains. Fixed!

NOT THE REASON YOU THINK - NOT a worn worm chain tensioning drive screw. It took a bit of thinking to work out why, but I got there in the end. My Stihl MS230 Chainsaw began to throw chains. It was missing a chain guide bumper. I often forget to put them back in when I have been cleaning and maintaining chainsaws, but I have never had one just fall out on its own.