With new cards being issued constantly, even for long retired players, it can be a daunting task. That doesn't even take into account the parallels, inserts and variations which already exist, but which a collector may not be aware of.
As long as I have been collecting cards of Herschel Walker, I'm not immune to making mistakes, in the hopes of adding a new and different card to the Hoard.
Here's an example: Below is a 1993 Pacific Prism card of Herschel
I have this card already and was perusing EBay one day when I saw this:
Wow, I thought. A cool variation of the '93 Prism card, with a way out, spiral design. I bought the card and when it arrived it looked exactly the same as the top card. I checked with the guys at the TCDB and to my chagrin, the cars are the same, the only difference is the camera flash.
I should have known better, but my desire for something different lured me in. Luckily, the lesson didn't cost me too much. Live and learn.