What Mistakes Did You Make As A Newbie?

In the mid-1970s, Rhode Island Rare Coin Investments was selling CH BU SLQs for $48 each. I bought one.

It was billed as FH, but it wasn't. Still, it is a beautiful coin and I really should have bought five of them. (Of course, I was in high school then and $48 was a ton of money to me.)

In my case, most of my big purchases - especially silver - came right after I got a particularly large amount of spending money (unexpectedly won an essay contest I literally found out about 15 minutes before closing time).

That was April 2011. Silver prices were up to $45 per ounce. I was buying for $40-45/oz (dealers didn't adjust the prices quickly enough, so what used to be huge markups briefly became technically under spot) and thought I was lucky.

I could probably count on my fingers the large (say, 5 gram or more) silver coins I bought in 2012 or later.

(Also, I bought a Peace dollar labelled 1921 without actually checking the date. It turned out to be an 1923-S in Fair. Still my only US silver dollar.)

Taking pricing advice from another website that said that some circulated clad coins were worth more than face. That site has reformed since then, so I won't bash the site. In general, if it seems too good to be true, it probably is.

I've made a few, mostly before the age of ten.
Buying low-grade or junk coins, like many others have said. I have a lot of common-date wheats and buffalos.
Buying whatever caught my eye at shows. I now have a lot of random coins that I don't really care about (I really need to get around to selling these).
Using the Red Book as a price guide. There was a time when I didn't realize it wasn't that accurate, so I overpaid on quite a few coins at shows.
Allowing relatives to buy coins as birthday/Christmas presents. I think some of them got ripped off on multiple occasions.
Selling to dealers, and not attempting to bargain. I could have gotten a lot more from some coins.
Spending all the money I brought to shows. I still do this.
A one-time mistake I made when I was very little was dumping a jar of coins into a Coinstar (or similar machine) without checking it first. I saw several mercuries slip away that day.
My biggest mistake was improper storage. I often kept coins loose, and at one point I got it into my head that 2x2s were only used when selling coins. I went to a coin show with my whole collection (which was small at the time) loose in a Ziploc bag, and a dealer literally came running out from behind his table to give me some 2x2s. On another occasion, many years before I had my safe, I had a great idea for how to protect my new purchase, an 1857 FEC- I put it under my bedpost with no protection.

One thing I haven't done is clean my coins. I learned from an early age that cleaning coins in any way is bad. Of course that's not entirely true, but it did protect me from some mishaps that certainly would have happened otherwise.

Biggest newbie mistake a bunch of years ago was buying 'shiny' wheat cents... I told myself the dies were probably at the end of life

When I first started buying I bought from an online dealer I used my debit card instead of waiting for a check to clear. I wanted my first shiny rocks quickly and patience is not a virtue of mine.

Biggest one I can think of is paying $81 for $10 face of LWC. The seller had a very recent feedback stating that someone found multiple DDO 1955 LWCs in their "unsearched lot" so naturally I had to buy a lot. I just kept bidding and ended up paying over 8x face for 99% 40s and 50s LWC. I got a decent amount in XF/AU but still. I still get a little sick thinking about that purchase!!