What Was Your Absolute Biggest Mistake As A Beginner?

"Those standard black plastic approval cards don't mix well with certain inks -- especially yellow and orange tones. IMO, they are OK for very short term use -- but definitely not for long term storage."

I only ruined a 638 that way, but I have seen tons of others ruined including expensive mint 287 & C1.

"Crystal mounts, bad."
evil
"Taping ends of Crystal mounts, bad,bad,bad."
This past week I looked at a collection where the collector hinged all the low values and put the high values in Crystal mounts and taped the ends. Many of the high values are now stained.