The two biggest blind spots are below and in front of you or above and behind you. My closest is the one I didn’t see but was told about it by a rather reputable pilot after the fact. I was flying along in a Nimbus 3 early on a 500 mile task out of Minden during the Open class nationals. We all started pretty much at the same time since it was such along task. This was before we carried transponders, had Flarm, or any of that good stuff. Anyway I zoom up in a thermal and I hear that my tail missed the guy directly behind me by about three feet! He has a cool head and isn’t known to exaggerate.