  There exists an upper limit on the mass of black holes when the cosmological
constant $\Lambda$ is positive. We study the collision of two black holes whose
total mass exceeds this limit. Our investigation is based on a recently
discovered exact solution describing the collision of $Q=M$ black holes with
$\Lambda > 0$. The global structure of this solution is analyzed. We find that
if the total mass is less than the extremal limit, then the black holes
coalesce. If it is greater, then a naked singularity forms to the future of a
Cauchy horizon. However, the horizon is not smooth. Generically, there is a
mild curvature singularity, which still allows geodesics to be extended. The
implications of these results for cosmic censorship are discussed.
