If the nexus is surrounded by minions, and you wish to autoattack a super minion for lifesteal with a + click while it is next to the nexus, the champion will ignore the minion(s) and basic attack the nexus instead. If I wish to lifesteal off a super minion to prevent death by ranged damage while I am destroying the nexus, I cannot attack minions that are too close to the nexus, no matter where on them I click.

Negligable but truly gamebreaking

If the minion is the closest thing to you and currently in range, a-move anywhere on the ground and you'll attack it.

The only game-breaking bug here is that you don't know to use your 'A' key.