The problem

Sirius asks Remus for a favor, but Remus has to promise beforehand that he will help Sirius regardless of what it's about.
Remus hesitates, but Sirius is his best friend, so what bad could happen?
Maybe Remus forgot that Sirius was the master of having impulsive plans and doing things without thinking about the consequences...
...maybe that would lead to a few problems? Or maybe even a few bigger problems? Or maybe this would be a key to their happy ending?