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 .

Spent so much time collecting the 6c, 5c, 4c, and 3c plate blocks. Wish I would have started with revenues, 1 and 3c 1851-57 and 3c 1861 a lot earlier.

I don't really have regrets from when I was a kid. That is what got me into stamps and a key motivator today.
I did use Crystal Mounts on a US Liberty but used and common stamps.
I do regret not getting familiar with the Scott catalogue. I had a US catalogue but never saw a WW list of stamps.

My biggest regret is when Steiner pages started, I invested in white pages thinking they were a good reference background. Now I prefer the traditional ivory like modern Scott Specialty pages. I have cut 80# white pages to Specialty size and still use those pages in Specialty binders.

Choosing the right page size, color, weight and mount is important. I value the presentation like a craft project.

My stamp buying strategy has also evolved. I realize that quantity of the most common really isn't a good start for those wanting to grow. In order to get the better issues, I end up buying collections that repeat most of the commons I alread bought.

Focus is an evolution. I see the value of more complete pages vs 100 binders with lot's of empty pages and spaces but I have enjoyed buying collections and having spaces in my albums. As a kid I always dreamed of having a stamp album that had illustrated spaces for all the stamps I owned.