Mistake or not, when you pull the trigger ( bow or gun)you are responsible for the results of that shot. I have been in the same situation where I could barely view a 4th point and had to let a buck walk not knowing if he was legal or not. Yes, it's hard but it's the rules. No, you can't shoot and count afterwards.

If it was me I would be a nervous wreck and would report it. But at the same time I don't think I would confuse a spike with a legal buck either.