The show is notorious for it.
In the pilot Charlie had been dead for 15 years. In the 'Job Hunting' episode of the same season he's been dead for 5 years
Job Hunting episode Blanche is relieved that she married money, in a season 5 or 6 epsisode she says all George left her were ugly suits
Season 2 episode The vacation written by Winifred Hervery Dorothy reveals Rose has had a nose Job but in Whose face is this anyway Rose says she's too afraid to have plastic Surgery.
Season 1 Sophia says she doesn't eat meat on Friday. Forgive me Father of season 2 she says she eats fish on Friday 'even though the pope told me otherwise'