Continuity mistake: During the classroom scene, the spelling of of Genghis Khan's name on the chalkboard changes from "Genghis Khan" to "Ghenghis Khan" between shots. (00:04:50)
Continuity mistake: At one point, when they time-travel away from "1,000,000 B.C.", Bill is on the left inside the phone booth and Ted is on the right. When the camera switches away, then back, they've changed sides.
Continuity mistake: When Napoleon first comes back from France and he's hanging in the tree, he disappears when the camera angle changes to an aerial view of Bill and Ted.
Continuity mistake: In the beginning, when Bill and Ted are at the Circle K and the phone booth lands with their future selves in it, you hear Bill say "Aw, I hate that part" just as they land in the booth. At the end when we see them land to talk to their past selves, Bill doesn't say that.
Continuity mistake: When the 2 girls at the mall leave after calling Freud a geek, the girl in pink puts her purse strap on her shoulder. In the next shot, it's off her shoulder, and she's seen putting it on it again.
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