Fields with super strict validation. For example, if you typed a postal code and "V5X4O4" works but "V5X 4O4" doesn't.

Icons with no textual label or title (on hover or not).

If a title is being truncated, I think you should be able to see a tooltip with the full title on hover.

When important buttons are obfuscated from end users in favour of advertisements or promotional material. Think of PayPal's "Continue without an account" button.

AUDIO THAT PLAYS WITHOUT PERMISSION! Lol - Also, no mute button.