Old ones.
When it was ~20 years old, my brother said their '78 Impala leaked everything a car could leak.
Somewhere beyond 500k miles, I thought my Mazda was leaking from the main seal near the crank pulley. I bought a seal they recommended at the NAPA store and replaced the old one. The problem seemed to be better at first, but then got much worse than before. When I showed the seal from NAPA to the counter guy at another parts store, he pointed out that it was a seal designed for grease, not oil, and just happened to have the right dimensions. No more leak after I installed the proper seal.