True. I suppose I was unconsciously thinking (now there's a phrase to fear!) about improbable dangerous events, where it is much more important not to underestimate P(X). If I get it wrong such that P(X) is truly only one in a trillion, then I am never going to know the difference and it's not a big deal, but if P(X) is truly on the order of P(I suck at maths) then I am in serious trouble ;)

Especially given the recent evidence you have just provided for that hypothesis.