Since voting varies so much by state, it may be hard to come up with an absolute error rate. The Al Gore election in Florida resulted in a number of 3%:
The National Opinion Research Center at the University of Chicago, sponsored by a consortium of major United States news organizations, conducted a Florida Ballot Project comprehensive review of all ballots uncounted (by machine) in the Florida 2000 presidential election, both undervotes and overvotes, with the main research aim being to report how different ballot layouts correlated with voter mistakes. The total number of undervotes and overvotes in Florida amounted to 3% of all votes cast in the state. The review's findings were reported in the media during the week after November 12, 2001.
Note that that would be on the extreme high side, as there were major issues with the ballots in Florida.
In other states, they have automatic recount laws if the margins are at a level they consider "within the margin of error". In Minnesota, for example, will trigger a recount if the margin is less than .25%:
For federal, statewide and district judicial races, a losing candidate can request a recount if the margin of victory is below 0.25% of the total number of votes counted or is ten votes or less and the toal number of votes cast is 400 votes or less. See MS 204C.35(1)(a)(2) and (b)(2).
So based on those two random anecdotes, the completely unscientific wild guess is that the margin of error could fall somewhere between .25% and 3%