Take the Oliver's New Castaways $5 TCR 8082. The TCR guide has it as a 'W' chip uncancelled, an 'L' chip cancelled with the overstamp, and a 'M' chip with the mark cleaned (reflecting the value of the sometimes tricky process). You can still see the remnants of the mark, either residual ink, the actual stamp indentation, or both.

I don't know the first thing about cleaning these marks off -- I bought them cleaned. Dealers are very upfront about this. Passing it off as uncancelled is WAY unethical.