This is definitely in jest and is most likely to be when I am providing a tutorial or at an early requirements gathering stage of a task, i.e. definitely development code or numbers scribbled on a napkin. I know humour is verboten at times on HN, but in some situations, e.g. say you have forgotten to close a tag in some plain HTML and the whole page goes table shaped, then you can lighten the mood and engage with people that bit more by feigning 'deliberate mistakes'.
With people that are not versed in how to test in a proper disciplined manner with some methodology to it there are occasions when you need them to focus more than they otherwise might. This could be important documentation rather than code. In these circumstances you can include a 'deliberate mistake' and set them the task to spot it. So long as you point out the answer if they don't spot it then all is good. This is harmless and is a good way of achieving the primary objective of getting your work double-checked outside of a formal testing environment.
This is a bit like the legendary M+M's in the rider that bands have. If the venue gets the M+Ms right then there is a good chance they will have got the other requirements of safety, etc. correct.