Final answer:
The least likely occurrence during operation of an engine equipped with a direct cylinder fuel injection system is premature combustion, resulting in uneven burning and knocking or pinging.
Step-by-step explanation:
In an engine equipped with a direct cylinder fuel injection system, the least likely occurrence is premature combustion. Normally, fuel is ignited by the spark plug, resulting in normal combustion where the combustion spreads uniformly outward. However, in the case of gasoline with an octane rating that is too low for the engine, it can ignite prematurely, causing uneven burning that leads to knocking and pinging.