The bug happened to me on summoners rift, once in a training game, and the other in a oneforall.
When leveling Akali's abilities I sometimes do not see the level going into the abilities and can level them up again, resulting in me at level 18 having everything maxed apart from w with only 2 points.
What were you doing when you found the issue?
I was playing normally just leveling abilities, if i recall I think I was in combat when leveling and I do not use my mouse to level, otherwise I am not too sure where this happened.
How can we test the issue?
I have no idea...
If I or anyone else finds this bug I am sure that they will try to inform you better on this bug, in the comments.