Did the speaker loose or lose his train of thought?
How to remember it:
Think of the related words lost, loser, and loss: they all have just one "o."
Loose and lose cause confusion partly because the spelling of lose is odd: it looks like it should rhyme with nose, but instead it rhymes with shoes. Loose, on the other hand, rhymes with words you'd expect it to rhyme with: goose, caboose, moose, noose (but not, of course, choose.)
Want more? Please see our list of Commonly Confused Words, Vol. 1.