For as long back as we can remember, people have been forgetting things. (Get it?)
One way that people help themselves remember something important is by tying a string around their finger (or something else) as a reminder.
The Butler tried doing the same thing, but to no avail. As soon as he would tie a knot, and an angel, a Malach, would come and untie it. No matter what kind of hint he would try to set for himself, that annoying Malach made sure he’d forget.
Lucky for Yosef HaShem remembered Yosef… No need for HaShem to tie any knots, after all, He doesn’t forget anything!