Tape
I've gotten PSA 2 with tape a couple times. Also got PSA 2 with a tiny bit of glue residue on the back. Another time I skillfully removed tape from the back of a T206, sent it for grading and still got a 2. PSA will grade a near mint card as a 2 if there's even a little tape stain on the back. They are very picky about that.
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