Bumblebee Tuna.
Ever since 1995 when Ace Ventura: When Nature Calls was released, I went around saying "bumblebee tuna" under the impression it was an original part of the script. I couldn't have been more wrong.
This startling fact was brought to my attention by Taylor at work, after his mom gave him a can of Bumble Bee brand tuna (as seen to the right). Further investigation showed that the brand has been in existence since 1899. They also sell Bumble Bee chicken, salmon, crab, oysters, clams, and sardines.
Youtube is full of Bumble Bee tuna related advertisements, including this vintage one, and all sorts of other theme songs. My girlfriend and I use "Bumblebee Tuna" as our guild name in WoW, but I had no idea it was such a huge phenomenon.
My life will never be the same.