clashing notecolumns

[John Galbraith]

What is a "clashing notecolumn"?  What features of a score make them,
and how do you find them in your input?  I get the feeling they are
rather benign, but they seem to be important enough to deserve warning
outputs.  I would like to fix my score so it doesn't have any "clashing
notecolumns" and no warnings.

Thanks,
        John