For me, I've had a float switch fail on for the ATO reservoir. It was the first time I've ever seen a mechanical float switch do this (I'm like the poster boy for gravity fed systems, until recently). I figured it wouldn't happen because it only touches RODI... boy, was I wrong!
To remedy, I basically set the ATO reservoir to auto-top-off, directly from the RODI unit, when 2 float switches are in the down/down position.
Then when both float switches are in the up/up position, a solenoid shuts the RODI unit off.
As a failsafe, I have a mechanical float switch inline so that it doesn't happen again. It's pretty cool really... I can feed all my tanks from a single 10g ATO reservoir if I want. Makes vacations more likely to happen now, too.
I plan on building a custom acrylic ATO reservoir here soon, to give me a little more time between the RODI going off and on. It works pretty good as is, though. I get about 2 days out of the reservoir now.
Another recent issue was my heater plug fused to my old Apex controller's inlet. It was destined to happen at some point. No losses or electrical fires (thank God). I wound up getting a separate controller that I have both heaters on, for the bigger tank. It doesn't touch my (nicer) reef controllers.