19

Deep Thought the computer made a huge mistake on the answer to Life, the Universe, and Everything. Turns out the answer is 19, not 42! This horrendous mistake was noticed when mathematicians proved that the answer to Life, the Universe, and Everything must be a prime number, which 42 isn't. As is well known, mathematical proofs, unlike scientific ones, can never be refuted. Ergo, it must be the case that Deep Thought made an error. It turns out some craphead programmer made a (serious) typo on the very first line of Deep Thought's code, for some reason causing the whole program to add 23 to the final answer. Only God knows what the heck when in that programmer's head at the time...