A thing, situation, etc. that is different from what is normal or expected (Oxford)
A person or thing that is different from what is usual (Cambridge)
The many anomalies in the tax system. (Oxford)
The anomaly of the social security system is that you sometimes have more money without a job. (Cambridge)
anomalies in the tax system
anomaly of the social security system
Abnormality
Irregularity
Sameness
Usual
Anomalies