A 172
The only reference I can find at all to "voluntarily failing" in the Rules Compendium relates specifically to saving throws:
The section on Caster Level checks makes no mention of intentionally failing.Originally Posted by RC p.112
There is a feat (Arcane Mastery, CArc p.73) that allows you to Take 10 on a CL check (which may, in turn, result in failure), but that's the only other option I am aware of.