Loose accounting is a mortal sin to the responsible adult world. If one really aim to be a serious, having-your-shit-down, reliable type of person, then you must devote part of yourself to being a bureacrat of your own life and track and tag all your transactions.
However, if you have no desire for such an attitude and sort of live in a constant maelstrom, you can often find funds that you completely forgot about right when you need it. In my case, I've had several miraculous deposit refunds when I was least expecting it and when I needed it the most (in one case I was about to have to borrow for rent when just in time comes a $400 check from the phone company for no apparent reason), forgotten stock investments yielding dividends you don't expect ("woah, I totally forgot that I invested $500 in AMD stock before the tech boom"), forgetting about the fact that you set a auto withdraw from your checking to savings account, friends that you wrote off as morally bankrupt deciding to pay you back, etc. In todays case, discovering that you have a mysterious and completely unused electronic gift certificate from a large vendor given to you by some friend (however, I can't remember who or when or even the fact that this incident occured), that you can use to regift so you don't look like a deadbeat to your nephew for not getting that christmas gift you promised him but forgot about until 4 weeks after the fact.