I recently ran a Pathfinder scenario for a group of new players who wanted to learn about the game. I was running Master of the Fallen Fortress. The thing required two medium sized spiders. So I dug through my Bones box and selected two simple ones. Painted half of them up and then I ran out of time.
I showed up to the game, ran them through it. The spiders had their moment and everyone agreed 'those are spiders'. I then finished them up at home and started searching the Reaper site for the code to post them.
It was then, and only then, that I realized these were not spiders, they are beetles! Doh! I painted them in a scheme that is about the same as the Pathfinder Bestiary spider... *shrug* Ah well.