He's about to reassure L that, except in the extremely unlikely event that they go to a party (that turns out to be incredibly boring) he will not spike any of L's stuff.

But then he meets Mello's eye, and begins snickering.

"Dude. You remember that time in LA? With the guy who -" laughing now, in earnest, gesturing vaguely at his chest, "- and the - with the--"

He can't finish. He's laughing too hard.