List rolls not accounting for weights
I use several different weighted list rolls in a game, and we had several instances of the rolls resulting in extremely unlikely results (as an example, we had one item, which should have a 1-in-40 probability, rolled 5 times in a row—a 1-in-102,400,000 probability! And then we kept rolling it!)
The results we rolled, looking back, were more in line with each item having the same probability, rather than accounting for the weighted list.