Other mistake: Beethoven didn't seem fussed nor resentful of Napoleon being part of the group. Historically he temporarily admired him and wrote the Third Symphony inspired by Napoleon's ideals of Europe's new hope of enlightenment but later despised him after he crowned himself Emperor of France in 1804 and furiously scribbled out his name off the title page of his Third Symphony and named it Eroica which he originally titled it as "Bonaparte Symphony" due to his former admiration of Napoleon.
Other mistake: At the Circle K when future Bill and Ted have finished talking to present Bill and Ted and go back into the phone booth, where are the other 5 historical figures? Especially as it is emphasized that they are all squished together, when they land you can see Beethoven, but he isn't there at the end when they leave.
Answer: While it's not unusual for musicians to try out new instruments (playing a few rifts and even entire compositions) in a music shop, Beethoven's extended sampling-keyboard performance went wild, drawing an enthusiastic mall crowd into the relatively small music shop. The shop manager no doubt felt overwhelmed and called in mall security to clear out the shop before any damage and/or theft occurred. Keep in mind that the security team was already scrambling to respond to several simultaneous disturbances throughout the mall, all caused by 7 strangely-dressed oddballs (more than half of whom only spoke obsolete dialects and ancient languages). The time-travelers were, thus, probably all perceived as one group of pranksters or escapees from a mental institution.
Charles Austin Miller