Anything that can go wrong will go wrong.
Corollary: At the most inopportune time
Extension: it will be all your fault, and everyone will know it.
Addendum: Anything that can't go wrong will go wrong.
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.
Extension: If there is a possibility of several things going wrong, the one that will cause the most damage will be the FIRST to go wrong
If you perceive that there are four possible ways in which something can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop.
Corollary: It will be impossible to fix the fifth fault, without breaking the fix on one or more of the others
Left to themselves, things tend to go from bad to worse
If everything seems to be going well, you have obviously overlooked something
Nature always sides with the hidden flaw.
Corollary: The hidden flaw never stays hidden for long.
Murphy's Law of Thermodynamics: Things get worse under pressure.
Matter will be damaged in direct proportion to its value
Enough research will tend to support whatever theory.
Nothing is as easy as it looks.
It always takes longer to get there than it does to get back.
Whenever you set out to do something, something else must be done first.
The legibility of a copy is inversely proportional to its importance.
The chance of the bread falling with the buttered side down is directly proportional to the cost of the carpet.
A falling object will always land where it can do the most damage.
A shatterproof object will always fall on the only surface hard enough to crack or break it.
A paint drip will always find the hole in the newspaper and land on the carpet underneath (and will not be discovered until it has dried).
A dropped power tool will always land on the concrete instead of the soft ground (if outdoors) or the carpet (if indoors) - unless it is running, in which case it will fall on something it can damage (like your foot).
A valuable dropped item will always fall into an inaccessible place (a diamond ring down the drain, for example) - or into the garbage disposal while it is running.
After you bought a replacement for something you've lost and searched for everywhere, you'll find the original.
No matter how long or how hard you shop for an item, after you've bought it, it will be on sale somewhere cheaper.
The other line always moves faster.