I just replaced the chain rings, chain, cassette and pulleys on my commuter bike. Given that only a few weeks ago I replaced its gear and brake cables, brake pads and handlebar tape, it's looking pretty good now, and won't need any major maintenance for quite a while.
As usual, I started putting the various bolts on the chain rings (for triple crankset) in the wrong order, and partway through had to undo these and start again, not the first time I've done that.
Then I had to remind myself where the guide pulley goes vs the tension pulley, and after checking a web page on the topic, realised it's pretty obvious when you think about their functions.