Short summary: When you have 3 ferocity on rengar, then jump out of a bush, cast q -> e in the air, you end up with 0 ferocity, but no empowered cast is performed.
1. Practice tool, black/gold Mecha Rengar skin
2. Refresh hp on, level 9 (refresh cd can be on or off), minions off, add gold x1, tiamat, black cleaver, cd boots as items.
3. Place enemy dummy in jump range of top middle bush
4. Jump on dummy with q-> w to generate 3 ferocity.
5. Walk back to bush and wait for q cd
6. With q and e both off cd and 3 ferocity, jump on the dummy
7. Both q and e should be pressed midair, with q before e
8. Dummy gets hit by unempowered q, unempowered e, + free auto reset that seems to come from using q in the air
9. Attack speed from empowered q NOT gained, dummy NOT rooted, so it is not just visual
10. After jumping on the dummy in this way, you are left with 0 ferocity.
11. So a full ferocity cycle has been completed without leaving combat, and without getting an empowered ability
The midair- q-> e combo feels good, however there is not only this bug, but also the inability to do this combo at full ferocity (which probably doesn't qualify as a bug?). If you need a video of me reproducing the bug I could make one, but I don't have one since recording is a bit of a struggle.
I submitted a regular bug report (sidebar) but I was not sure whether I should do this also. Also posted a link in the rengar changed thread just in case!