I know people that put french fries in their burger, but i don't really consider that weird
sometimes I put potato chips or pretzels in my sandwich to make it crunchier
I used to mix ketchup and mayo and dip my pizza in it...I think that's probably the nastiest thing I've done.
Other than that I mostly just use ketchup for dipping (grilled cheese + ketchup
...I don't think that's weird either though)