I had not either until recently. I had purchased 6 or 7 rear main seals during covid as I had three engines going together and always something in the works. Well my 64 had a small leak from the rear main, not a huge deal but I HATE leaks. I did not build the engine in the car originally. So I pulled it apart to look it over and fix the leak. Going back together I used one of the seals I bought in 2020. Put it back together as I always do nothing different. It went from a small leak to a gusher!!! Driving it around the block I saw it loose all pan vacuum and by the time I hit my driveway I was leaving a trail. The seal I pulled out of it was the same color and likely brand as I used. So I ordered a new seal from another manufacturer and no issues. So looks like I will throw away the remaining seals from 2020 as that is the secind one that leaked. My Predator had a small leak as well after being on the dyno and a few passes on it.