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We use Local Group galaxy counts together with the ELVIS N-body simulationsto explore the relationship between the scatter and slope in the stellar massvs. halo mass relation at low masses, $M_\star \simeq 10^5 - 10^8 M_\odot$.Assuming models with log-normal scatter about a median relation of the form$M_\star \propto M_\mathrm{halo}^\alpha$, the preferred log-slope steepens from$\alpha \simeq 1.8$ in the limit of zero scatter to $\alpha \simeq 2.6$ in thecase of $2$ dex of scatter in $M_\star$ at fixed halo mass. We provide fittingfunctions for the best-fit relations as a function of scatter, including caseswhere the relation becomes increasingly stochastic with decreasing mass. Weshow that if the scatter at fixed halo mass is large enough ($\gtrsim 1$ dex)and if the median relation is steep enough ($\alpha \gtrsim 2$), then the"too-big-to-fail" problem seen in the Local Group can be self-consistentlyeliminated in about $\sim 5-10\%$ of realizations. This scenario requires thatthe most massive subhalos host unobservable ultra-faint dwarfs fairly often; wediscuss potentially observable signatures of these systems. Finally, we compareour derived constraints to recent high-resolution simulations of dwarf galaxyformation in the literature. Though simulation-to-simulation scatter in$M_\star$ at fixed $M_\mathrm{halo}$ is large among separate authors ($\sim 2$dex), individual codes produce relations with much less scatter and usuallygive relations that would over-produce local galaxy counts.

Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies