Vi's E sometimes won't proc if you have Guinsoo's
I'm not saying it won't proc double, since it's not an on-hit and isn't supposed to. But sometimes it just won't proc at all. It'll go on cooldown, but won't do the added damage or the cone. Whether or not it's actually on the Guinsoo attack.
You can see on the left that I activated my E. And even despite the fact the next attack was an in-between, not an actual Guinsoo proc, neither the bonus damage nor the cone happened. But it still went on cooldown.