Speaking of collectibles, I had three; comic books, cards, and coins.
The coins were probably my first foray as I inherited my dad's coin collection when he died (I was 2 at the time). The comics and various cards (baseball, soccer, comics, etc) were late 80's and early 90's where I wasted an inordinate amount of my childhood savings. Worse is I almost never read them, I just collected.
Turns out they all ended up being worthless. By the 80's, I think everyone had caught onto the whole, "If I buy this and keep it in mint condition, it'll be worth a fortune!"
The coins were probably the biggest slap in the face, as I realized that the only ones actually worth anything are things like mis-prints. And by the time I was even born, everyone knew about them. Worse, my wife worked at a bank and they allowed her to "buy" (trade in) quarters that she wanted. She did this for a few months grabbing the pre-1962 quarters. I think a year or two ago I went through the entire coin collection and her quarters. The silver value of her quarters made them actually valuable; about $800. The rest of the coin collection? I might be able to get $50 if I could unload them all.