Key group is not in KeyGroupRange

[junjie.m...@goupwith.com]

hi:
本人遇到了这个报错：
Key group 51 is not in KeyGroupRange{startKeyGroup=64, endKeyGroup=127}. Unless 
you're directly using low level state access APIs, this is most likely caused 
by non-deterministic shuffle key (hashCode and equals implementation).

这个报错出现的莫名其妙，想了解这个错误是在什么样的情况下才会导致这个报错的？
谢谢！！