I think I've posted this before, but it seems relevant:
Back in the early eighties, I worked on communications links for radio repeaters for public utilities and emergency services radios.
We had a rural repeater site on top of a hill, which had a microwave link to another repeater site on another hill about ten miles away. in between the two were a number of other ranges of hills, but all just low enough that line-of-sight was maintained.
This microwave link worked fine for a number of years, and then suddenly began to fail intermittently but always around the same time on a Thursday afternoon. It didn't happen every week, but say every two weeks.
After a lot of investigation, and a complete replacement of equipment at both ends, it was discovered that on one of the intervening hills was a small country lane which went over the brow of the hill more or less in line with the line-of-sight link.
Off that lane was a sort of lay-by or turning circle, which was used by the local bus service to turn round at the end of one of their routes.
The buses would drop off at their last stop lower down the valley, and drive up and turn round, and then wait for half an hour before starting the next run.
Turns out that most of the time, they used a single decker bus on that route, but Thursdays were market day, so they used to put a double-decker on the route on that day. If it happened to park at a certain spot in the lay-by, it used to neatly break the line-of-sight between the two repeater towers.
The only reason we found out about this was that we set up a temporary intermediate link (a van with a couple of dishes on it) in that lay-by whilst we were testing, and the bloke saw the bus come and park up whilst he was there, otherwise we could still have been looking!
The microwave antenna on both of the masts was at the very top of the mast, so couldn't be raised, but we managed to build an extension for the antenna on the other mast, and re-align the path just enough that it would be above the possible obstruction.