Walking on someone's line of putt.

If he does so intentionally then it's a breach of rules too !

Decision 16-1a/12 Player Walks on Line of Putt

Q.A player walked on his line of putt. Did he incur a penalty for a breach of Rule 16-1a?

A.Yes, if he did so intentionally. No, if he did so accidentally and the act did not improve the line.