If anything can go wrong, it will.
If anything can't go wrong, it still will.
Nothing is ever as simple as it seems.
Everything takes longer than you expect.
Whatever you set out to do, something else must be done first.
You can't do just one thing - there are always consequences.
If everything seems to be going well, you have obviously overlooked something.
Nature always sides with the hidden flaw.
Mother Nature is a bitch.
It's impossible to make anything foolproof, because fools are so ingenious.
Never lose heart - they might want to cut it out.
You can fool some of the people all the time and all of the people some of the time, and that's sufficient.