I often use cider vinegar on chips in England.
Peanut butter on apple slices is surprisingly nice.
Apple with peanut butter is very good!
When my daughter was around two, she spent a week eating pineapple with parmesian cheese. It’s actually not too bad, you get the sweetness of the pineapple and the salty of the cheese.
Balut and spiced vinegar. The spiced vinegar cuts through the creaminess of the duck embryo. 
One hot summer day after coming home from a long hike, I couldn’t decide between treating myself to ice cream or a beer, so I had both. They’re surprisingly good together.
Liver-sausage on cinnamon raisin bread.
A favorite since childhood, & popular among some Eastern European immigrants & their descendents.
I’ve flogged this here before: peanut butter and Dijon mustard on pumpernickel bread. Soooooo yum!
An ex introduced me to canned pear halves with the hollow filled with mayonnaise and topped with shredded cheddar. I thought it sounded awful, until I tried it. Soooooo yum!
My friends and I got into the habit in college of eating Chic Fil A chicken biscuits for breakfast with a pack of grape jelly on it. Sounds gross, tastes yummy!
My father saw my wife eating her eggs that way, and was revolted; I asked him why; he thought about it, and realized it’s because he’d routinely used ketchup to make decidedly gross eggs into barely passable ones when he was in the Navy.
The conversation inspired him to try ketchup on perfectly good eggs, which he found surprisingly delightful.
I had no idea ketchup on scramble egges was considered so odd.
I used to like dipping my pizza crusts in cherry pop. I haven’t done it in ages, so I’m not sure if I’d still enjoy it.
Ground pepper on Strawberries. Brings out the flavour beautifully! Strongly recommended.
+1 on the tartar sauce with french fries.
For those who like ketchup on scrambled eggs, try cocktail sauce (or that red chili sauce you can get in sushi places - sriracha) some time.
I also like grape jelly mixed with my cottage cheese.
Pizza ‘bones’ (the hard rims) dipped in blue cheese or Italian salad dressing. Not so odd, since Italian restaurants sell breadsticks to dip in olive oil and such, as appetizers… When we had bacon and eggs and toast years ago, we ate the toast with grape jelly. Had a fried egg sammich last week with grape jelly and I felt like I was 10 again!
I used to do something like this as a kid. Pizza hut breadsticks dipped in Pepsi. OMG that was good.
My niece just posted on Facebook that she has recently eaten mashed potatoes with Nutella.
She said it was pretty good.
Tuna salad sandwiches with salsa. Yummy.
100% of everything mentioned so far sounds disgusting to me. Fortunately, being a vegetarian, I have an excellent excuse to not have to taste any of it. 
PB and salami too. I’ve even had PB and anchovies. It’s amazing how many things go with PB.
As a complete carnivore, I have to say that pretty much anything purely vegetarian is a gross food combo to me.
Vegetables add extra flavor to meat as far as I am concerned. 
Apples and pb
Fries and vinegar
Cottage cheese and peaches?