Currently, there is no ability to un-report a post which you have reported. However, you can suggest this feature to Fandom Staff.

If it happens, just notify the mods or admins that it was an accident.

Usually, it's pretty easy to tell when someone reported something by accident and I just ignore it

You'll probably only face reprecussions if you mass report random posts for no reason, though

If we are talking about Discussions, just because something is reported means nothing towards what an admin or mod will do with that report. As a mod, I often just clear reports (aka Approve), if there is no obvious problem. People make bogus reports all the time.

As said above. I know on here, I'll go through and review the post and the context before I approve, delete, or lock something. If it was an honest accident, that's fine. We can usually tell.