It means you are not 100% stable. It is very rare for all cores to fail together. Each core is a bit different even with them on the same die.
This means you need to clock down a bit or add a very small bump to the voltage.
When testing overclocks, also ensure that the memory is at default speeds and not overclocked. You do not want memory effecting your tests. Once you find a stable point, then you can test memory at higher speeds if you wish.