A local gravel grinder came in with his bike thinking all he needed was a new chain and cassette.
Wrong answer. As all good mechanics know, you've got to check out those chain rings as well. Many times they also need replacing. This one was glaringly obvious.
Just replacing a chain, at the proper time, can be a good way to go, but once everything wears beyond a certain point, you are going to have to replace the drive train as a whole: Chain, cassette, and chain wheel(s). This fellow obviously uses his big ring as his main drive gear, but I also replaced his middle ring, as it also showed significant wear, just not as severe as the big ring.
Sure, it cost him a little more, but he wouldn't have been able to pedal one revolution with just a new chain and cassette without the chain "skipping" over that waaaaaay worn out big ring!