Yes, there are some combinations you should never try because they can actually do more harm than good.
Pineapple and milk: Pineapple has an enzyme called bromelain which is very healthy but have it with milk and it leads to nausea, stomach ache, infections and headache.
Fruits and vegetables: Don't mix these two because they digest differently. While fruits are quickly digested, veggies take time and together they cause diarrhea, infections and stomach pains.
Guava and banana: This combination can give you gastric issues, acidity, nausea and persistent headaches.
Orange and milk: They play enemies in your stomach with orange's acidity delaying milk's digestion!
Papaya and lemon: A highly dangerous combination for children especially, it can lead to anemia and hemoglobin imbalance.