So I picked up 
with the Arctic Warfare skin... From time to time I could see the auto attack animation, but it dealt no damage. This was happening mostly when Headshot was about to charge. Also a friend of mine played with the Sheriff skin on... Sometimes he could not see the auto-attack animation, but he dealt damage.
So this bug has been around for over a year - as a cait main, i never thought it was that bad (she was the first champ I got mastery 5 on). It has been pretty noticeable lately though - I am asking one of our teams to investigate and see if we can reliably find how to reproduce the issue
Followup Q, was lulu in the game with you when the issue occurred?
Happened to me on Headhunter skin....static shiv would proc before the aa animation starting. Same with crits. Something is very wrong
I noticed this issue when I played her as well.
I was using the Sheriff Caitlyn skin, and it seemed like the damage was being applied right as the bullet left the gun, as opposed to when the bullet hit ~50% of the time. Made it very difficult to last-hit in the laning phase.
Match history of the game the bug occurred in for information's sake: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1921682634/32643352?tab=overview
There was a Lulu on the enemy team, it looks like.
Same thing happened to me when I was playing 
,I'd auto attack but no animation would happen but i'd still do dmg
