I bought my current cell phone in mid January 2008, which means I have had it for ~830 days.
I tend not to lose sight of the few things I keep on me at all times - Keys, wallet and cell phone. In this 830 day period, I doubt I have forgotten my phone for more than three days. Ok, lets just add some extra five days, for times I have travelled without the phone charger, resulting in me not having a phone available. So, eight days out of 830 gives us that the probability for me accidentally not having access to my phone for a whole day is close to 0.01, or 1% ⇒ P(stupid) ≅ 0.01.