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.

Left my nascent plate block collection out one weekend on the kitchen table while I was out at Sunday school. My grandmother thought she wss helping me out by removing the selvedge....When I got home I was so upset I almost cried (I was 8), and made a fuss. Now when I think back on it what really makes me feel bad is how I must have made my grandmother feel.
Oh yeah, and the other mistake was collecting plate blocks.

Buying stamp from a retail stamp store . The dealers sold me a bunch of U.S. at a great discount of 50% of catalog he said he was helping me out . All the stamp were seconds or maybe one step above trash . Straight edges, heavy cancels thin spots . I learn what those were at a stamp club months later .