For example: I don’t believe in the axiom of choice nor in the continuum hypothesis.
Not stuff like “math is useless” or “people hate math because it’s not well taught”, those are opinions about math.
I’ll start: exponentiation should be left-associative, which means a^b should mean b×b×…×b } a times.
Some of the more complex proofs might be wrong just because so few understand them, and the ones who do might have made mistakes.
Hell, I’ll trust a math result much more if it’s backed up by empirical evidence from eg. engineering or physics.
Don’t know if that counts as being ”in math” by OPs definition.