a dripping faucet, for example - is it dripping at 10 drips per hour? or 5 drips per hour? did it stop dripping? no way to tell - the period between drops could be one moment longer than you observed - it could had dripped right after you stopped observing it.

this upper limit is called the busy beaver function. but it is only applicable to turing machines, which we don't build, because we don't have an infinite tape.