The more embarrassing it is for a bug to appear in the product you are demoing the more likely it is to occur. Conversely, and also as common, during a demo of a bug in order to get help in fixing it, the more embarrassing it is for the bug to disappear, the more likely it is to happen.
Note, you cannot use this law to fix bugs.
The CEO of your company walks by your desk and asks you to demo this great product you are working on and are ready to release. It is nearly guaranteed to fail and show a bug.
A week later, after battling this bug and holding up the shipping date, you call the CTO to come in late and help you, and he brings the CEO as well. It is nearly guaranteed that the bug will disappear.
If, however, you attempted to use this law to fix said bug, and called the CTO in order to demo it with the hope it will dissapear, then both the original bug and a new set of bugs will appear.