Consistent Prefix Read
A guarantee that if a sequence of writes happens in a given order, a reader sees a prefix of that sequence — never an out-of-order subset.
The anomaly it prevents is the one that makes systems look broken: a reader sees the answer to a question before the question. Formally, if writes W1 then W2 occurred, no reader may see W2 without W1.
It matters most in partitioned systems, because a partitioned log gives ordering within a partition and none across partitions. Two causally related events written to different partitions can be consumed in either order, and a consumer that acts on the second without the first produces exactly this class of bug.
The fix is usually the partition key: route causally related events to the same partition — key by conversation, by entity, by aggregate root — so ordering is preserved where it is needed and parallelism is available everywhere else. Where that is not possible, the events must carry sequence numbers and the consumer must buffer.