"stumbled upon" = "discovered by accident" "Stumble" is usually used to describe an ungraceful motion like nearly tripping over a small pebble in the road. People stumble upon family secrets or stumble upon solutions to problems. In a worst case scenario, "I stumbled upon my best friend and boyfriend kissing each other." >_<

One can also say "stumble on" or "stumble across". It means to encounter or discover something (sometimes someone) by accident. "Scientists searching for a cure for the disease stumbled upon a new form of adhesive".

I stumbled upon this question and decided to answer.