Rehearsal is not the time for you to practice your part. Practice at home. By the time you show up to the second rehearsal you should be able to play your part. Rehearsal is for getting all the parts working together like a well-oiled machine. When you come to rehearsal unprepared you are wasting everyone's time. And that's rude. All the people who came prepared now have to just sit there and twiddle their thumbs while you poop your pants in front of them.
Rehearsal time is for the collective issues, like intros, transitions, endings, getting dynamics and tempo changes together, and learning the art of the piece, not for you to learn the friggin notes. You should come there with the notes down cold, or at the very least have a way to hide your lack of courtesy on the passages you can't nail.
The vast majority of these groups give you paper or digital copies of the music to take home with you, and it's not to line the bottom of your birdcage with it, capisce?
"Oh, but these groups are just for fun!" I get that, but trust me, it's a LOT more fun to sound good and have productive rehearsals than it is to sit there and listen to the second clarinetist figure out which end of the horn to blow in.
"Oh, but he's 75 years old and will never be able to play the part, and this is the only fun he ever gets to have." Okay. But if you must have people like that, at least hide them in some lower part and don't waste rehearsal time with them. In fact, for folks like that, why even have rehearsals? Just have weekly barbecues for socializing and let them all sight read the concert. Apparently it won't make any difference in the performance.
I suppose this mostly applies to amateur and community groups. If you're a professional and this happens, chances are you won't be professional for very long. Heck, in many professional situations you might not get a rehearsal, or only a short rehearsal the day of the gig. I remember getting hired for a famous TV awards show, and the only rehearsal we got before playing for millions of people was about 45 minutes to run intros and transitions. They can get away with that because the musicians take pride in what they do and make sure they can play the stuffing out of their part before they ever show up.
You might not play as well as that pro, but you should take everyone else's time just as seriously as he does. Now close your computer and go practice.
That's my rant!

Ready...aim...fire!

(P.S. This is all intended to be tongue-in-cheek. Sort of.)