Even a Broken Clock Is Right Twice a Day - Phrase, Meaning and Origin
A broken clock is unreliable as it cannot properly tell the time. So whenever you look at it, the time it shows is going to be wrong. However, even a busted clock that has its hands stuck in place will still be right two times a day, hence the clock is correct occasionally. This is similar to a person who, like an unreliable clock (in that they often give wrong information about something), they can still be correct at times.
This phrase goes back to at least the early 18th century. It was used in a magazine called The Spectator, by Joseph Addison and Richard Steele, from the year 1711:
"If instead of running after the mode, they would continue fixed in one certain habit, the mode would
come time or other overtake them, as a clock that stands still is sure to point right once in twelve hours."
KnowYourPhrase.com - Common Phrases and Idioms with their Meanings and Origins
A person who is considered to be unreliable can still be right about something every once in a while, even if it is by accident.
Example: When I was having car trouble, my friend, who knows next to nothing about cars, guessed that the problem was a belt on the car's engine. After taking it to an auto repair shop, it turns out that my friend was correct, but 'even a broken clock is right twice a day.'
This phrase is also said as 'even a stopped clock is right twice a day.'
Note: Did you know that when it comes to the origins for many phrases, that they are unknown? Indeed, and in cases where this happens, I'll usually do either one of two things: If there are explanation(s) that exist that try to explain how a saying came to be, then one or two of these might be included on the page. Or if not that, then a quote will normally be included. These are typically the oldest known quotation of the idiom in print.