Pistol magazine springs.

[Jack]

It's long been conventional wisdom that springs will weaken or 'take a set' if kept compressed in a full magazine for long periods of time.
Finding an actual example where that has happened with a modern magazine is a bit more difficult- usually someone's cousin's barber's next door neighbor's best friend told them about it.
There are examples of loaded 1911 magazines from WW2 or even WW1 still functioning fine.
If you're worried about it, rotate your full magazines every few months. Or, just don't worry about it.