Would you be willing to share a moment of failed logic or come up with one?

Not sure where I remember this from but -

“If one man can dig a post hole in 60 seconds, then through logic two men can dig the same hole in 30 seconds. Given that increasing the work force to twice its size effectively cut the amount of time it takes to complete the job by half, we can say that 60 men can dig a single post hole in one second…”

- this is just an example, and as one (with common sense) can see, it is rather fail…