Believing bi people are just confused, and will eventually end up being straight or gay.
Believing bi people can not be loyal partners because they will cheat with other genders.
Believing a bi person is no longer bi if they marry or commit to a serious relationship with a partner.
Believing bi people are "half gay" or "half straight" instead of just bisexual.
Believing someone isn't truly bi until they've been in a relationship with both genders.
Believing "everyone is a little bi" and therefore underplaying the discrimination bi people face.
Believing bi people have an unfair advantage over lesbian and gay communities because they can "pass" as straight more easily or that bi people are "not queer enough" to belong in LGBT spaces.
For more on the topic of bi-phobia and bi-erasure: