Things happen, people DO have bad days, sometimes it's like a
case of Murphy's Law - everything bad suddenly happens when it's
most vital that nothing goes wrong. Haven't you experienced such
pure suay luck? 
It can put
your best laid plans to nought.
I've seen candidates who present themselves well, give brilliant
answers to all the questions posed, but after hiring them, we
realised that it's just a nice veneer.
On the other hand, there were people who appeared a tad unpolished,
'clumsy', not-too-confident, etc who we have hired, and who turned
out to be diligent employees who deliver good results and who
improve with time, like aged wine. 
So, you can't really form a harsh judgement on a candidate just
because of something as frivolous as not bringing along a pen. Oh
well, different strokes for different folks!
