Here's a mistake:
Let's suppose I went to the local grocery store and opted for an inexpensive alternative to my usual premium brand--one pound. One taste of my morning coffee, and I would know I had made a mistake.
Now comes the sin considerations.
If I looked to the Bible for guidance, I'd read about "doing unto others as one would do to oneself." Possibly that means either throwing away (or giving away) the unsatisfactory coffee one purchased, or going back and purchasing or using the unsatisfactory coffee for a few weeks until it's used up. That's a reasonable expression of the Christian way as expressed in the Bible.
Alternatively, one might look for the easiest way to correct the mistake of the least consequence to oneself. The most obvious is to go back to the store and steal one. That's a sin--it's also a crime--and you might be scared off by the possible consequences: risk/reward.
Avoiding obvious risks, one could keep the unsatisfactory coffee, buy and use the preferred for yourself, and serve the unsatisfactory coffee to your unsuspecting visiting friends--a sin that only you would know about.
One could get sneakier. One could bring the coffee back and ask for an exchange--claiming that something was wrong with the coffee. Now, that would be a lie, which is a sin(different from stealing). Or, one could truthfully admit that one made a mistake, asking the shopkeepers if they would take it back anyway. That might not be a sin if one did it infrequently; but if one did it regularly, with a strategy to make it less noticeable to others, it would hop back into the sin category--wouldn't it? Only you could make that determination. Nobody else would know--maybe.
This "sin stuff" gets very complicated very quickly--and even you yourself might not know if you were doing the right thing and avoiding sin--or not.
One needs help. Help in achieving discernment for oneself; help and guidance in living the right way.