It was for sure not a Democrat that found this “miscalculation” in Trump’s favor ...

6000 won't suffice.

For Wisconsin:
https://twitter.com/NationalistTV/status/1323927447449051137?s=19

A mistake ok.. but is it a common mistake?

It usually yes when voting is still going on. Stuff like this happened in 2012 and 2004, when Bush VS Kerry ( Bush camp tried to contest a state Kerry had won) and Obama VS Romney both ran close,so there wasn't an early winner where the % difference was higher than remaining uncounted vote %. Tight races take longer for obvious reasons.