We point out a new mechanism which can lead to mean field type behaviour in nonequilibrium critical phenomena. We demonstrate this mechanism on a two-dimensional model which can be understood as a stochastic and non-conservative version of the abelian sandpile model of Bak et al. bak. This model has a second order phase transition whose critical behaviour seems at least partly described by the mean field approximation for percolation, in spite of the low dimension and the fact that all

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... ons are of short range. Furthermore, the approximation obtained by replacing the lattice by a Bethe tree is very precise in the entire range of the control parameter.