mismatches & mutations

bioquest

please, could someone tell me, is a mismatched base considered a mutation, some or all of the time. somebody kind of confused me on this, they were answering my question through email but Im confused about this one thing
thanks

Monique

It is a mutation, but it can still be repaired by the mismatch machinery if the abnormal base pairing is still present (before DNA replication).

bioquest

I understand you cant see which cells have mutations without killing the cell but why cant you xray the cell or something and see if it has a mutation that way (without killing it) I mean you can xray through solid objects
?
is it because there would be too much movement or something?