I am still so green at the orchid hobby, but my worst offenses so far have been:
1) using the bark mix straight out of the bag, which did not help the hydration of the plant
2) watering overhead and getting droplets trapped in the crown, which seeped down into the lower leaves. Luckily, I only lost 1 leaf from this rookie mistake.
I am sure there is more to come, but that's it so far
Well, unless you count my very first orchid ever (that was 20 years ago). Oh boy, I rotted those roots REAL good!