In the universe of Liar Liar, curses do happen - they're rare, rare, rare - but there are documented occurrences.When Fletcher and Greta are joking about the 24 hour curse going around, they're not really acting as if its totally a joke. Then Greta takes Fletcher at his word that he's cursed, both in walking out and then forgiving him. Fletcher can't use his inability to lie due to a curse as an excuse to postpone because the legal system has seen it before and prefers people to testify that way anyway.
The reason Max's sixth birthday is such a subdued affair compared to his fifth is because Fletcher is still trying to get back on his feet after losing his job a year previously.Big party for fifth, and just parents and a cake for the sixth. Jerry must have picked up a lot of the tab last year.
There was no curse.Moved from Head Scratchers: My fan theory (to this lie and to the whole film actually): No magic spells, Fletcher was unconsciously unable to lie (with that one exception) due to his guilty conscience catching up with him. When his son told him about the wish, it just solidified the psychological effect, and gave him a point in time which his conscience could let him off. This adds a plausibility to the story that a magic spell would not allow for.
A minor tear in the fabric of spacetime......took Mark Bellison's inability to lie and put it on Fletcher Reed. It closed before Mark could get his back and evaporated withing 24 hours for Fletcher.
Max lied when he said he wished for rollerbladesHe really did wish for his parents to get back together, he just didn't want to tell them that. It would be very ironically funny.
The curse was actually forever, not 24-hoursAlthough Max wished that his dad would be unable to tell a lie for 24 hours, the wish was actually permanent. Notice how after the 24 hours are over, we never see Fletcher lie again. By then Fletcher is comfortable with telling the truth. Fletcher doesn't even tell any lies as a test to be sure that he's capable of lying again.