Microsoft had a bit of an issue with the latest round of patches last week. Nothing major blew up other than an Outlook folder pane view, but the implications were legion. Do you actually test these patches before you deploy them?
Microsoft isn't the only culprit, though they are the most visible. Application vendors, operating system vendors, and even game companies have been bitten in the past about rushing incomplete patches out the door and causing more trouble than they tried to fix. That's one of the reasons why many companies have started using the model of one patch day per month.
Patch management software is critical in this regard. Robust patch management allows you to designate a testing environment to ensure that things won't crash when you apply the latest security fix. It's important to provide an additional level of confidence because your environment could be considerably different than the reference design.
Vendors can't test everything. There is a limitless combination of programs and file versions that exist on any number of systems in a typical enterprise. All a vendor can do is ensure the patch won't destroy the base program while fixing the problem. It should be up to the admins and technicians inside the customer environment to test the patch with the mix of software and hardware that they have installed.
Good patch managers will give you every opportunity to use good practices to keep your systems from crashing when you deploy a new set of patches. Using those features is up to you.
Have you had a patch go wrong? Did you test it first or did you rely on the vendor's assurance it was okay to install? If so, are you going to test those patches in the future?