Oh, this one's easy...
"Glocks don't blow up" and when they do, "It's the ammo's fault." Never mind the fact that we were getting regular NCIC teletype bulletins on these occurrences from the time the .40's came out; never mind the fact that you can go pick up brass behind a .40 Glock, and find a high percentage of seriously-bulged cases. Never mind the fact that the 'offending ammo' works fine in other guns; never mind the fact that there are now a good number of non-LE shooters who have either experienced a blow-up first hand, or seen it occur on a public range.
The faithful wail, "It's not really happening! You're making it up because you hate Glocks." Right. Kind of reminds me of when we were potty-training the kids, and they used to tell us that somebody else had peed their diaper.
Glocks are not what I hate. What I hate is the fact that the manufacturer has produced a moderately-dangerous product, refuses to address the problem, and the buying public (including the idiots that make procurement decisions for LE agencies) just keeps right on patronizing them, without requiring that they fix the problem.
Sorry, but you asked.