First and most common is people levering on the brittle Iridium electrode to gap them. this is BAD and can cause you to lose the end in use.

Second most common is over tightening. You would have to look at a cross sectional view of the Iridium plugs to understand that even slight overtightening can crack the ceramic core insulator. This results in one of two things:

1) The center insulation material breaks away in use.
This can destroy the engine if your unlucky.

2) The cooling efficiency is badly affected, resulting in plug overheating. End result can be detonation or mechanical plug failure.

The plugs have a very specific tightening torque and going past this almost always damages them. If your unsure, have someone else fit them.

Finally, a lot of old school mechanics grease the threads of the plug and this has an effect on the plugs cooling ability. The cooling efficiency is badly affected, resulting in plug overheating.

There are other minor things , but those are the ones we see here every single week. There are way too many people out there who think they know what they are doing but cant even install a set of spark plugs correctly.