I'm going to post the absolute dumbest "repair" that I've ever had the pleasure of doing. But maybe it will help someone.
Last summer, I replaced the coolants in my RAV4 EV. I was careful about purging the air from the blue coolant system (thanks Alflash!) but I guess I wasn't as careful when I added the pink coolant. That coolant is used in the HVAC system and is recirculated with a pump that you can see just inside the front grill if you look straight down. It's facing up at you and is different from the other pumps.
Anyway, I didn't realize that I must have left air bubbles in the line. Recently, I'd very occasionally been hearing knocking coming from under the hood. It would go away after maybe 10 seconds. What made it suspicious was that it happened yesterday when I was stopped at a red light. An EV only has two types of moving parts when it's not in motion: fans and water pumps. So I popped the hood and saw that the pink coolant was gone from the reservoir.
I turned off the cabin heat and topped up the pink coolant when I got home, making sure to run the defroster to keep the water pump running at its maximum. It took a few fill-ups of the reservoir and I had to burp the cabin heater water pump line a few times to get the air out of the system. I left it slightly over the fill line and when I got to work it was slightly below it. So far the knocking hasn't recurred.
I'm posting this because if I made this mistake then maybe someone else might, too. This could have been serious. I imagine that I could have burned out the cabin heater system. I was probably saved by the fact that I seldom run it.