I took my RO apart to put a new DI canister on it after breaking the old one.
Any way put it back together, switched on and tested water to 000, went out left it running all night filling the tank in my van. Next morning I checked the TDS (don't usually do this) only to find it running near 500, checked the water in the tank, near 500 too.
Turns out I had put the waste pipe into the DI, was polishing the water down to 000 initially, but soon knackered the resin, had 1000ltrs of useless water and a day off.