To reproduce:

1. Place a button and press it.
2. Break the button and place another in the same position.
3. Press the button again.

Steps 2-3 must be done before the button's normal pressed time has passed (1.5 seconds for wood, 1 second for stone).

The button you place must also be the same type as the first (e.g. oak).

The button "unpresses", or goes back into an unpowered state, after the amount of time has passed-- but relative to the first press, before the button had been broken, causing the new button to remain pressed for a shorter time. This bug can interfere with certain redstone features, especially with commands where a button is immediately reset.

Sorry if I made that hard to understand:

A wooden button will unpress 1.5 seconds after it was pressed, even if it was broken, replaced, and pressed again in that time. A stone button will also have this problem, but will unpress after 1 second.