Since 2000, seven out of seven peer-reviewed academic studies confirm that Democrats tend to make more mistakes than Republicans. The populations that are, in academic jargon, "vulnerable to error" are low-income, low-educated and minority, all of which disproportionately vote Democratic.
So Scott Rafferty concludes this makes recounts favor Democrats, particularly in states with systems like Minnesota's:
The largest source of error is going to be human error -- marks that the machine cannot be expected to recognize. The voter crosses out one oval and fills in another. The machine sees an overvote, but the hand canvass counts the vote correctly. When the voter circles the candidate, or uses a check mark, or doesn't fill in the entire oval, the machine sees no vote at all, but again, the hand canvass registers the vote.