。
An unforgettable incident occurred.
Literal
Forget-[hard-to-がたい] incident [subject-が] happened
忘れがたい uses the literary suffix ~がたい (hard to do), formed by attaching がたい to a verb stem. Related to the more common ~にくい but distinctly more formal/written; typical in news, essays, and set phrases like 忘れがたい経験 (an unforgettable experience).