An interference engine is one in which the valves, when at their maximum
open position, protrude into the cylinders to the point that the pistons
could hit them, IF the pistons were at top-dead-center at the same time.
The timing of the valves is such that this never occurs during normal
operation. However, it can occur if the timing belt breaks, slips or
loses enough teeth to throw the valve timing out. It can also occur if
the timing belt is replaced incorrectly (with the valve out of time) or
possibly in the case of a broken valve spring that doesn't close its
valve quickly enough.