In an ideal situation a three phase load should be evenly distributed between the phases. This all too often is not the case.
A small voltage imbalance will cause a high current imbalance, overheating would then occur in one or more of the phase windings of a three-phase motor. In some cases the overheating is serious enough to cause motor windings failure due to insulation breakdown.
A single-phase load connected, to a phase, which is out of balance with the other phases will also be subject to heat related problems.