In the season 5 episode "The In-Laws", Mark says he has never met Vanessa's parents but her mom was in the season 1 episode "In Vanessa We Trust".

Vanessa's mom was played by Roxie Roker in the season 1 episode and by Dolores Robinson (Holly Robinson Peete's real mother) in the season 5 episode.

Roxie Roker passed away in 1995 so they had no other choice but to get another actress to play her mom but they shouldn't have pretended like she never visited in season 1.

Another case of bad continuity. Luckily, this show didn't suffer too bad from this TV disease.