There is one I learned early on that I remember, because I learned it the hard way.
I had some large (GML) script for a GameMaker game. I hadn't indented it, because the code ran the same way anyway.
Some joker friend removed a } when I was away from the computer. I found out not too long after, and put it back.
Then I spent a number of hours over the next week trying to find why my script didn't do what I wanted. Only after a week did I find out that I had put the } back in the wrong place...
I've indented my code very well since then.