Jenny Calendar's computer is a generic unbranded PC. And yet when she hides the "Translator Pro" program from Giles you can clearly see an Apple OS desktop (probably System 7 since OS 8 was released in 1997, the same year as this season).
Willow sprinkles fish food into her aquarium, yet doesn't seem to notice that there are no fish in there.
Jenny asks Angel how he got in, and he says the sign outside invited him. However, they are at the school which is a public place (and a place he's gone to many times) not a private residence, so he doesn't need an invitation to come in.
Willow says that she is nailing crucifixes to her bedroom wall, but she is nailing a cross, not a crucifix. The distinction being that a crucifix has the body of Jesus on the cross. This could be explained as Willow being Jewish and not knowing the difference, but she is shown as being very smart and studious so she would know.