I once failed a job interview because a unit test I'd written produced different results for one of the interviewers than it did for me. I didn't understand why, they didn't understand why, but it was enough to end my chances of joining that company.

Today, around 2 years later, I realised what the cause was and it all seems so incredibly obvious.

Not only is there a huge amount we don't know, we also don't know what we don't know.