As far as I know potatos contain harmful toxins so are a big no no for Guinea Pigs.
And as previous poster said, Iceberg lettuce as it can give them diarrhea.
Found this on line, HTH:
what not to feed your guinea pig, no matter how good it may taste to you.
While guinea pigs do enjoy a variety of flavors and types of fruits and vegetables, there are some foods that you should never feed to your guinea pig. If there is an item that isn't on this list or the one above and you're not sure, it's best not to feed it to your piggie until you can get verification from a vet. The good foods list isn't comprehensive, so you may find some other online resources with more exhaustive lists.
The following is a list of food items to NEVER feed to your guinea pig.
? Iceberg lettuce - this won't kill your guinea pig if you give him or her a little piece, but it has virtually no nutritional value, is quite fibrous and watery. It's been known to give guinea pigs diarrhea. With the great selection of other more leafy, dark lettuces available, just avoid this one and go with one of the others.
? Any type of cabbage - like iceberg lettuce, a small piece is not likely to severely harm or kill your guinea pig, but it's very gassy and not good for their little systems. Stay away from it if you can.
? Cauliflower - also very gas-producing.
? Potato peelings - not that this would be something you'd be likely to give your piggie anyway, but toxins can exist in potato skins, particularly if they've begun to turn green, and they can be poisonous. Just toss them in your garbage disposal or trash instead of your piggie.
? Raw beans (this doesn't mean green beans. This means things like raw, hard kidney beans, split peas, pinto beans, etc.)
? Shelled nuts or seeds, particularly things like sunflower seeds or 'bird seed' type mixes, pumpkin seeds, peanuts, etc.
? Rhubarb - the leaves of the rhubarb plant are poisonous, and the stalks are very, very sour.
? Any type of meat or fish. Guinea pigs are vegetarian and looove a salad over a steak any day.
? Dairy Products - while many stores will offer 'yogurt drops' and other 'treats', dairy is not healthy to feed your guinea pig and can actually cause problems. Feed natural treats like veggies instead of store brand treats. It's less expensive and your piggie will be much happier.
? Chocolate, coffee or other caffeine-containing products.
? Alcohol (this should go without saying!!)
? Cookies/bread*
*you can feed a small piece of cereal like a cheerio from time to time as a treat, but they will likely not even like it, so it's best to just avoid this altogether!
? hamster food, gerbil food, rabbit food, or any other non-guinea pig food. Often these contain seeds, which are a big no-no, but they also have a different balance of vitamins and minerals which aren't suited for a guinea pig's dietary needs.
? corn kernels. Popcorn isn't a very good idea either. Guinea pigs choke easily and think of how easy it is for us humans to get popcorn kernels stuck in our mouths and throats!
? sugary foods like donuts, candy, or soda
? Ice cream.
? Any processed or fried foods that you would eat, including potato chips/nachos or french fries.
? Cheese
? anything spicy. jalapeno peppers, anything with cinnamon, etc.