As the repair tech at our LHS, there are many people who overfill smoke units and then bring them back to the store dripping with smoke fluid. They almost always have loose or missing tires, even if the engine was new.
However, many times, its not the customer's fault. Many Lionel diesels drip smoke fluid inside their shells, frames and trucks, no matter how carefully you fill the smoke unit, and regardless of how little liquid you use. The worst offenders are Lionel's poorly designed smoke units utilizing "smoke manifolds" (as they are called on the gensets), or "funnels" as they are called on F3's or other diesels. Liquid soaks that little gasket between the manifold and shell, then just keeps sheeting down the inside of the shell, all over. Many engines also profusely leak where the manifold enters the smoke unit. Anyone with a Lionchief Plus FT knows what I mean - you'll be familiar with smoke fluid slime dripping down your trucks onto the wheels.
I have repaired many of these leaky diesels by coating the perimeter of smoke manifolds with a bead of "Seal-All" adhesive, which resists gas, oil and most solvents, like smoke fluid.
Nobody wants grimy smoke fluid on the track!