This is in fact a small mistake, but considering the cameo-effect it is a rather big one:

In Season 2, the episode where House is caught at his door by an hiv-patient, you can clearly see the sign with the number "221B" at the house-front. This is a reference to the legendary Sherlock Holmes, who resided at the same house-number in London.

In the episode when Wilson comes home (to House's apartment) he finds the stethoscope at the door-knob indicating House is with a woman inside. So Wilson stays outside, waiting. For around 30 seconds, one can clearly see the sign at the door, showing "221" without the B.