I bought a yugioh 20 card pack and i found a used card in it. is this normal?

I am new to yugioh and I bought to packs of 20 to make a deck from scratch (I know I should of got a starters deck but i wanted to make it from scratch). Any way, in one of the packs i got a normal monster card with worn corners and a F written in marker on the back. I don't really care because i replaced it with a card from a booster pack. But I was just curious about why it was in there and is it normal?