Murphy's Law

or,

Nothing is as easy as it looks.
Everything takes longer than you think.
If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong.
If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way will promptly develop.
Left to themselves, things tend to go from bad to worse.
Whenever you set out to do something, something else must be done first.
Every solution breeds new problems.
It is impossible to make anything foolproof because fools are so ingenious.
Nature always sides with the hidden flaw.
Mother Nature is a bitch.