I had that happen to me when Lynn and I were down in Cabo. My ball landed on the green and made a real deep mark, and stayed right there in it's own crater. Looked rather strange seeing my ball half buried in the green like that. But, as Bill stated, it's okay to fix ball marks, even those you didn't make. Just don't repair spike marks, as that's not okay.
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Putting is easy, IF you have the right putter.
Later, Don.