One which, unlike D&D OE, AD&D, AD&D2, BX, BECMI, D&D Rules Cyclopedia, D&D 3e, D&D 5e, and presumably D&D 4E, is unforgivingly deadly.
Rests are vital, but note the prohibitions on repeats...
Warn players about dump-statting Con - it's not just a bad idea, it's a near guarantee of short lived...