LFS's do have a bad rap, as there are some good ones, but MANY of them do give bad advice(the one down the street I have heard many times giving bad advice)
Also for the RODI, I use and egg timer for sprinklers with a much higher success rate(still had the hose come out and dump water all over my garage)
Some I've made over the years and not previously stated:
Not monitoring levels..at all
Dumping salt straight into the tank
using tap water for top off
not acclimating corals/fish
"forgetting" to do cleaning maintenance