General:
* Odd
* Uncommon
* Bizarre
* Unorthodox
* Peculiar
* Unusual
* Out of the ordinary
* Extraordinary
* Unprecedented
* Remarkable
* Notable
More specific:
* Mysterious (suggesting an unknown cause)
* Eerie (suggesting a unsettling or spooky atmosphere)
* Anomalous (suggesting a deviation from the norm)
* Aberrant (suggesting a deviation from what is expected or normal)
* Outlandish (suggesting something exaggerated or ridiculous)
* Surprising (suggesting something unexpected)
* Unforeseen (suggesting something that could not be predicted)
* Uncanny (suggesting something unsettlingly familiar or strange)
Figurative:
* A bolt from the blue (something unexpected and sudden)
* A twist of fate (something unexpected that alters the course of events)
* A once-in-a-lifetime event (something very rare and memorable)
The best word to use will depend on the specific context and the feeling you want to evoke.