Liar Liar/WMG

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 Headscratchers:

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.