Little smooth bivalves
Back when I was an undergraduate, I did a project on the taxonomy of freshwater crabs in Lake Kivu, one of the Rift Valley lakes in sub-Saharan Africa. I had plenty of experience describing and differentiating species of African freshwater crabs, so my advisor decided I was ready to take on the project. There was only one problem: all the crabs looked the same. It's not that I was inexperienced - far from it - but for whatever reason, in Lake Kivu, all the species of crabs lost their defining spiky features and became little smooth crabs. I eventually found a few structures that differed between the species and wrote reliable descriptions, but the process was far from easy. Then during Covid, when I started foraging in the woods around Massachusetts (partly as an excuse to get out of the house and partly as apocalypse preparation), I came across countless non-descript mushrooms. My identification guide warned against picking "little brown mushrooms" with no distinct feat...