While we're waiting on Waterfreak:
"It happens to the best of receivers, too."
In the case of a repeater the highly selective "cans" pretty much eliminate the receiver, since a great many use commercial transceivers and the rest use specially made repeater transceivers mixing in the front end is pretty much out of the question.
In my previous post I was also thinking of dissimilar metals and rusted towers, point contact rectification. I remember two classic cases, the first being a State Police tower site that was a real hair puller. It took months to isolate some "bad connections" and corroded tower joints, hardware was replaced and the joints jumpered over, problem solved. Some years later a similar problem was found and rectified (;->) at a water tower where a local 2M repeater is located.
As an aside, hangar 1 at Lakehurst NAS is familiar to most being the site of the Hindenburg disaster. What many perhaps don't know besides it's still in use as an airship hangar is the fact the loft is stuffed with radio equipment from soup to nuts, everything from Navy to federal to a local fire company, even the Jersey Shore ARC has a packet node/BBS and a 70cM repeater up there. The roof is an antenna farm par deluxe, I wonder how they manage to keep it all straight. The view from up there is as fantastic as the radio range, 200' above flat terrain sure is an advantage!
Now here is a giggle for you, every once in a while the Amateur packet station goes off the air and it's always the same, a power failure. Here's the giggle, a technician working up there will trip over the power cord for the rack and pull it out of the wall socket and leave it lying there on the floor. It's pretty aggravating getting our techs who have security clearance together and go there just to plug it back in. There is a stupidly simple solution, replace the plug and socket with a twist lock type but there is a catch 22, no problem with the plug but the socket is Navy property! THOU SHALT NOT TAMPER WITH NAVY PROPERTY for I am a jealous god and thou shalt have no other gods before me. There is an Amateur station in the radio room of the USS New Jersey moored at Camden but I digress....... the Navy going to war with the Navy?