From what I've heard, the instances Midler has been calling for line have been because of tiny tweaks that have been made to the script throughout previews thus far. It's not that she's having trouble remembering the script in general- it's that she's having trouble reconciling what she's already memorized with the new changes that are going in.
For what it's worth, two separate friends who've seen the show (neither particularly diehard fans of hers) both said they were so engrossed by her performance that they didn't even care when she went up. I'm sure she'll be ready to go by the opening at the end of the month.
Tonya Pinkins: Then we had a "Lot's Wife" last June that was my personal favorite. I'm still trying to get them to let me sing it at some performance where we get to sing an excerpt that's gone.
Tony Kushner: You can sing it at my funeral.