Extension has a very questionable interaction with abilities that alter an attribute of an existing effect, like killing a mob with beserking. This increases the duration of the effect by 30 seconds.
Ik that it could be argued that this interaction isn't really a big deal, but there's a mod i play with (copper pots lol) that has adds a lot of foods with the ability to extend effects. Extension makes them extremely op, basically increases the time that these effects gets extended by 30.

idk if i explained it properly, here's a video showcasing the bug https://cdn.discordapp.com/attachments/729290878652252161/881797652637970452/pextension.mp4