Pilots mistaking an ATC " expect FL XXX " for a clearance to climb to FL XXX are unfortunately not that rare especially amongst Korean and Chinese crew.
I had personally encountered two incidents, one in Manila airspace, the other in Vietnam airspace. On a dark night a Korean Air flight asked to climb up to FL370. MNL ATC replied " standby for climb to FL370 ". Korean flight readback " roger, leaving FL350 climbing FL370 " That was stepped on by another transmission from another aircraft and MNL ATC did not pick that up! Luckily another Korean Air flight with a Malaysian or Singaporean English accent picked up on that and transmitted immediately to the offending KE flight on 121.5 that he WAS NOT CLEARED AT ALL to FL370. The offending flight crew sounded confused until a Korean pilot who was on the other KE flight manned by the Malaysian/Singaporean crew came screaming in Korean lingo, after which they sought verification from MNL ATC who categorically chastised them that THEY WERE NEVER CLEARED FL370!
The other incident involved a Chinese carrier in Vietnam airspace where a similar mistake was picked up by ATC radar.
The two incidents snuffed out my earlier plans to join any Korean or Chinese carriers.