I used to drop the chain on my first-generation Langster; it happened because the nuts next to the adjustable cones on the axle deformed the aluminum dropouts, allowing the axle to slip forward no matter how tight the outer nuts were. I threw on a banjo axle adjuster I had left over from my first track bike (got it in 1966). Then I bought a Felt TK2, which has steel-reinforced rear dropout faces.