When Brandon is talking on the phone, the top of the microphone is visible for about 4 seconds.
When Brandon is calling Melissa to get a date (at about 6:45 in) the shotgun mic drops into the shot.
In the previous episode, Brandon lost his license and "can get it back for work in 3 weeks". While on the show more time could have elapsed, he's already driving on a date.
Brandon hadn't told Brenda or Kelly the name of the girl he met at school, but when Kelly calls the number Brandon gives her, she asks to speak with Melissa.
The boom mic is showing when Brandon calls Melissa to ask her out. It happens 6 minutes and 21 seconds in.
Second time boom mic appears at 34:25 minutes