Fruit + Food combining
Fruit should ideally be eaten away from other foods. Fruit goes through the digestive system very fast (in as little as 15 mins), compared to say a potato (1h), or beef/pork (3/4 hours).
So if you combine them together, your digestive system struggles, you either get improperly digested food going through the system quicker than it should, or fruit sitting in the stomach for longer than it should. This causes them to essentially ferment/rot in the system, causing digestive issues. This also causing the release of some nasty chemicals which we don't want hanging around in there.
Some specifics and exceptions:
Melon should always be eaten completely alone, away from all other foods!
Lemon, lime and tomato (acid fruits) combine well with green and non starchy vegetables
Nuts, oil-rich seeds, cheese, yogurt, kefit, and other fermented dairy foods (which are high-fat proteins) combine well with all acid fruits (lemon, lime, grapefruit, orange, tomato, strawberry, pineapple, pomegranate and kiwi)
In general try space fruit consumption out to an hour before meals and 2 hours after.