Not specific to China Southern. It happened at Korean Air too. Local crew. Captain in the toilet. F/o heard clearance to climb to 9800m but was actually only cleared 8900m. F/O read back 9800m but ATC controller did not catch that mistake. Climb was initiated to 9800m set on the MCP. Passing 9300m, alert controller caught the level bust and ordered flight back to the correct flight level 8900m which the F/O hurriedly complied with. The crew apologized to the controller. As there was no conflict with any other traffic in the vicinity, the Chinese ATC controller decided to let the mistake slide and declared " no problem, no report to CAAC "
The Captain taught that it was the end of the matter and did nothing. The F/O decided to be super honest and went to verbally report to the chief pilot as some kind of precaution to other pilots of Chinese ATC not hearing or monitoring readbacks. Some kind of sharing unfortunate incident with lessons to be learnt.
The ambitious chief pilot took it upon himself to investigate, even communicating with the Chinese ATC to verify that the incident took place. The Chinese ATC confirm the incident but as no conflict with any other traffic took place they would not take it any further. However the chief pilot blew it up, initiated a safety violation enquiry insisting the Safety Department take punitive measures. End result, both pilots were grounded for 2 months with requirement to attend numerous technical and disciplinary inquiries, with the punitive measures of having to undergo several long winded safety classes.
So such sh
it happens in Korea, China and I believe in Jan and Taiwan too!