I kept playing the same melody on guitar and couldn't figure out what it was...turns out it was the main theme from The Lord of the Rings movies.
A few years back, noodling on my bass in my basement (bassment?), I came up with a great riff, and played it for two minutes solid before I realized: it's AC/DC, Hells Bells.
What's really silly: that riff was my ringtone at the time, and it still took me 2 minutes to recognize it.