My B&M Fly was knocked a few times and eventually the thin, brittle plastic cracked around the mounting hole. I repaired it with superglue and it worked for a while until it got another whack then broke off FUBAR. I replaced it with a B&M Cyo which has a much lower profile and stronger construction.
The Fly is just very vulnerable. Depending on your bike geometry, it is possible to whack the Fly when your handlebars flop to one side. On one of my bikes, the Fly would pass under the top tube, on another, it would bash the TT.