If the engineer does not take quick corrective action (i.e. by closing the regulator) the locomotive can end up stationary with its wheels spinning: this can damage both the locomotive drive mechanism, through running too fast; and the rail surface, which, in extreme cases, can be left with a dip where the spin took place.
Also, the effect is much more likely to occur with a heavier train or where the rail surface is compromised, for example, during wet or icy weather, or when there is oil or leaves on the track.
Ideally, locomotive designs will have roughly equal power-to-weight ratios that enable smooth acceleration from a 'cold start', or stopping position.
However, if the power of a locomotive vastly exceeds its weight, then an imbalance ensues which causes the violent spinning of the wheels through loss of traction.
Other causes include the contact of oil with the flanges and rims of wheels, which reduces adhesion with the surface of the rails, and a general loss of traction on steep gradients when pulling heavy loads.