It happend allready in a few games of mine ( don´t have footage because I didn´t thought of it in that moment)
The problem is that after you E as Rengar to get your passive stacks loaded sometimes (didn´t check it throughly enough)
you get your stacks ready but the E (Bolastrike) is still on the normal cooldown instead of beeing up for the snare.
Would be nice if you could check it, you just have to look if:
After you stack to golden with Rengars E if there is still a cooldown, if thats not the case im sorry for bothering.