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Texas A&M alumnus accidentally joins band, says he pretended to play for 4 years
(www.lonestarlive.com)
When I was in college band (UW Husky Marching Band), we would regularly march "packers," as in a person who doesn't play the instrument but was filling that instrument's hole. Drill would be written for a certain number in each section, and if it was easier to have one less tuba than have a hole in the piccolo section, a tuba player gets to march piccolo that week.
It was the easiest week of marching I had all season, and the flutes had some fun dances for me to learn.
Long story short, though, any given week there would be a few people just holding an instrument and marching, so one guy doing it every week wouldn't be a big problem. Really wonder how sectionals went, though, as that's just the section in a group playing the music. Tough to fake that.