General:
* Abnormality
* Deviation
* Irregularity
* Exception
* Outlier
* Oddity
* Peculiarity
* Aberration
* Discrepancy
* Inconsistency
* Fluctuation
* Variation
More specific:
* Rarity (if it's something rare or unusual)
* Curiosity (if it's something interesting or puzzling)
* Mistake (if it's an error)
* Defect (if it's a flaw)
* Mishap (if it's an accident)
The best synonym to use will depend on the specific context of your sentence. For example, if you're talking about a scientific observation that doesn't fit the expected pattern, "abnormality" or "deviation" might be the most appropriate. If you're talking about a strange event that's hard to explain, "oddity" or "peculiarity" might be better choices.
Think about the specific meaning of "anomaly" in your context and choose the synonym that best captures that meaning.