Some years ago three Spanish pilots flew their aircraft, one after the other, under the Ballachulish bridge. At least one person has flown under the Skye bridge and just last year there were reports of two light aircraft having flown under the Kessock bridge.
IMO none of these things would have been particularly hazardous, though illegal, and in the case of the Kessock bridge you could probably get a 737 through there had you a mind to.
I've been tempted but cameras are too ubiquitous these days so that the chances of getting away with it are probably quite minimal.