I think there are things that individual humans will never understand because we have finite lifespans and finite memory capacity. Anything that requires more storage space to understand than a human brain possesses will never be completely understood by a single unaided human. We might be able to build a system that includes at least one human that does understand it (in the style of Searle's Chinese Room); after all, a human with enough time, enough paper, and enough pencils can indeed execute any Turing machine, but I wouldn't want to claim that "I" understand everything that the program I'm running does.