When Thea talks to Death, her undershirt switches from green to blue.
When Bill and Ted encounter Death in Hell, he is playing tetherball. After B&T distract him the ball hits Death on the head. They cut to B&T and then when they cut back to Death he is standing by the tetherball pole but no ball is hanging from it.
Around the 20min mark Bill has his shirt tied round his waist and then suddenly he's wearing it and takes it off.
The Rufus hologram mentions first meeting Bill and Ted in 1989. Though Bill & Ted's Excellent Adventure was released in '89, it was explicitly set in 1988.
Babe Ruth is shown batting right-handed during the famous "called shot". Ruth actually batted left-handed.
Mozart was only 5ft 4in, but is seen as tall as Jimi Hendrix, who was 5ft 11in.
There are no examples in all of Hendrix's official or bootleg catalog that reflect the ability to play anything resembling Mozart. That kind of electric style did not appear until the arrival of Eddie Van Halen in the late seventies. Hendrix's playing was far looser and he rarely used his little finger for anything but chords.
This movie's set in 2020 but the Great Leader mentions Bill and Ted did their concert 25 years ago meaning that Bogus Journey would have to be set in 1995. Bogus Journey inconsistently set itself in 1991 and 1993.
Bill and Ted are essentially morons, having nearly flunked out of high school. Despite this, when Ted looks at the watch, which reads 6:19, in a split second said that they had 58 minutes left which means that he either knows the math trick or he is really fast with math.
While Electricity wasn't discovered and made usable until after Mozart, Jimi leans into the time machine to grab a patch cord. The cord itself leads back inside the time machine. Jimi is therefore using the time machine as an amp, which would have its own power source.
During the final scene, as the countdown from ten reaches seven, all five actors on camera visibly and incorrectly mouth the word "six" while the word "seven" is very clearly and correctly heard.
While future Bill & Ted from 2025 are urging their 2020 counterparts to join them on the patio of their mansion, future Bill can be heard shouting, "Hello! Younger us's, hello!" but his lips never actually move.
Bill & Ted realize that in order to unify humanity with their song, they have to have everyone that ever lived to play an instrument, yet people in the present, as well as those in the future are shown not playing instruments.
Mozart seems to not understand English and speaks only in German, but actually Mozart spoke and wrote in English, he had spent 15 months as a youth living, composing and performing in London and was also taught English by his father.
Billie and Thea travel back to 1967 to recruit Jimi Hendrix, and Billie tells him how great Electric Ladyland is. The girls are portrayed as having an encyclopedic knowledge of music and musicians and should therefore know the album would not be released until the following year.