Prue claims that in the future she has an "amazing" new office at Bucklands, but it is clearly the same office that she uses in 1999.
When Piper is tracing where the door will go, she draws a line well above her head. When the new door opens it is a different shape and much smaller than she originally traced.
When Prue is looking at the "to create a door" spell, there is nothing between the pages. Seconds later, as she's flipping through the book, she finds the prison map inserted between the pages where the "create a door" spell is, where there was nothing just moments before.
When Leo is confronting Piper and Prue in the town square and they zoom in on the switch hunt poster, a character who is supposed to be frozen clearly blinks.
Future Prue and Piper are flicking through the book of shadows looking for the Return Spell to get back to there current time. They say its not in there, but Prue Actually flicks past the spell clearly written in the book twice in the episode.