This has happened to me more than once. But I've learned my lesson and speak up that I'm not splitting, I'll pay for my share. And even if I don't have cash, I announce how much I'll be putting on my debit card.
But this was a new one for me...just a couple of weeks ago I went out with 4 other friends to a nice Italian restaurant. I ordered a butternut squash ravioli that everyone LOVES (but no one else ordered). There are only 5 large ones for $17 in the portion. But they are very rich and I could only eat 3 and was going to box up the other 2 and take them home. I started to hand my dish to the waiter to get boxed up and 2 of my friends were like "you aren't going to eat those? We'll eat them!" And the waiter handed my dish to them. I was just so shocked, I didn't know what to do. I mean, how awkward for me to say "no!" So they ate 40% of my meal. Ugh. Oh, and one of them boxed up the remainder of her meal since she was full after eating mine.
Live and learn, that's for sure! It's just sad that I have to be so cautious now!