Continuity mistake: In Bridget Jones' Diary, Mark says that he was 8 and Bridget was 4 when they would play in the pool together. but in Bridget Jones' Baby Mark sets up the compatibility profile and his age is 50 and Bridget's is 43.
Other mistake: When Jack confesses to Bridget that he lied to Mark, there is a landline telephone on the table beside her. When Bridget goes into labour and Mark throws away his mobile, they wonder how they will phone for a taxi, ignoring the landline.