Most ID numbers for an airplane at most companies are simply the first 3 digits of the registration number which of course is on the tail, but also usually on the nose gear doors. The mistake can happen as easily as any, you check a sheet and it says plane "701" and you go out in the cold and see "601" and mess it up and get in. Trust me after a couple minutes you realize the mistake or someone points it out to you, it never gets to the point you go off flying the wrong airplane, nor is it a sign of a bad pilot. Imagine you are a cop and get assigned a different car each day... someday in the lot you will try to climb in the wrong one, it is just bound to happen.