This is for an upcoming article about the flaws of each princess film. Your comments may be used for the article

I wonder if Erci had kissed her before the third day, would she have her voice back? But it's not an inconsistency

This is the first movie which leaves the frame of a fairy tale and tries to be more a fairy tale based story. It's overall very flawless. The only thing which makes no sense is Sebastians line about Ariel staying mute until the kiss. The contract said "voice for legs", the kiss only allows Ariel to keep her legs, not to get her voice back.
Oh, and Ariel is a little bit dumb that after the desaster with the fork and the pipe during dinner, she still uses a fork as a comb before she goes to bed.

If Ariel knew how to write why didn't she write to Eric? It's not really a flaw, maybe she just didn't think about it. The movie would have been duller that way.

I don't really see any problems with this film, except for Ariel's character, and her ridiculous relationship with Eric

There are some animation inconsistencies in this film, for example, Ariel going from being under the covers in one shot to not being under the covers in another (or maybe it's the other way around.) Also, Sebastian has a thimble on one of his legs in one shot when he finds Ariel after Part of Your World, and in the next shot it's not there. In the commentary, the animation crew said they were aware of this, but didn't have the budget to go back and fix it.

^ The movie was made when their films were not bringing in a lot of money, and the studio did not have much money. Ariel looks like different people a few times, but nowhere near as inconsistent as Belle is