Anti-pattern 1: Auto-incrementing ID as shard key
Problem: All new writes go to the highest shard. Creates a write hotspot.
Fix: Use UUID or hash the ID.
Anti-pattern 2: Timestamp as shard key
Problem: Recent data is always hot. One shard handles all current traffic.
Fix: Use composite key (user_id, timestamp) or random prefix.
Anti-pattern 3: Low cardinality field
Problem: status field with values creates shards max, with uneven distribution.
Fix: Choose a field with many unique values.
In interviews, after proposing a shard key, proactively mention what could go wrong.