Yup, and although the prior poster misunderstood the meaning of the term "harmonics", the overtones caused by other transmititters will also do exactly the same thing in combination.

We've been able to document one such repetitious occurance at our field that will hit planes EXACTLY every 15 minutes.

A frequency checker shows no activity while the planes exhibit "hits" on the particular channel.

Needless to say, that is now a "banned" frequency.