In pile-up free experiments there is only one muon candidate which is unambiguously identified by muon tracking. The accidental background thus arises from accidental electron candidates. In experiments with pile-up (i.e. several muons in the TPC), the dominant background comes from accidental muons in the target TPC, whose electrons are binned relative to uncorrelated muons by the reconstruction program. The accidental level achieved in both situations depends on the details of the cuts and the method used muon-electron vertex tracking. For the second case the source rate for accidentals is much higher requiring stringent vertex cuts.