Conjecture Distributivity of the lattice of funcoids (for arbitrary sets and ) is not provable in ZF (without axiom of choice).
A similar conjecture:
Conjecture for arbitrary filters and on a powerset cannot be proved in ZF (without axiom of choice).
See this blog post for a rationale of this conjecture.
See here for used notation.
The first conjecture is shown false (that is a proof without AC exists) by Todd Trimble.
* indicates original appearance(s) of problem.