Garen is unable to crit immediately after proccing Rapid Fire Cannon
Just tried it in a custom game. I had 100% crit and if you press Q and it procs
then Garen's next auto attack will not crit. If you auto attack first, then use Q, then Garen will continue to crit at 100%. If you just auto attack without using Q at all and it procs rapid fire, then Garen will continue to crit. The bug is just when you use Q to proc it.
I had 1.5 attack speed and when rapid fire would proc on Q, his attack speed on his next auto attack felt like it dropped to 0.7, but this auto attack did crit. But when the attack speed immediately after using Q felt like it was at 1.5, then this is when it would not crit.
So things to check are: Why does does his attack speed feel like its lagging and why is he unable to crit if there is no lag to it.
You are not going to have 100% crit on Garen usually, but being unable to crit on an auto attack seems unfair.
Sad day in Critmacia.

I have not tested it on other champs that have displacement abilities such as Udyr and Trundle, but I would think that they would have the same problem.