If its the occasional accident, clean it up and move on. If its constantly happening, where were you when it happened? If you were nowhere to be found, then blame yourself because supervision is key. If he continually does it in front of you without warning, then a vet visit is called for - that may signal and underlying health issue.

What might help is if we knew what you do now so that we can tell you what you're doing right and what you might need to change. A lot of people do something to sabatage potty habits without realizing they are.