When IT makes a mistake, it is usually felt by a large number of people and is very visible. IT departments know this and don't like to make mistakes. Thus, you have the reason so many IT departments say NO or move slow.
Even when they say no or move slow, mistakes are bound to occur. Systems are incredibly complex today and no amount of testing by users (and IT) will catch every scenario as the systems get more complex.
Leaders need to support their IT departments and accept a reasonable amount of challenge in any project. I'm not saying allow for careless IT, just that a reasonable amount of challenge is to be expected. The bigger and more complex the project, the more likely you will find challenges.
The type of challenges you should tolerate are the situations that aren't obvious or that are difficult to test. The types of challenges you should be less tolerant of include things that are obvious or easy to have tested. Before you get too upset, be sure that you as a leader asked the right questions and had the right people involved. If you rely on IT to figure every scenario out, you will have problems. Business and IT have to work together.
When business and IT work together, you will reduce the amount of errors in a project. Plus, the business is now invested in the outcome. While some business users will still complain about IT after being appropriately involved, the business recognizes they had a chance to ask questions.
Leaders who complain loudly about IT undermine the ability of IT to properly support the company. If your IT department is making lots of obvious mistakes, it is probably time for an IT leadership change. Instead of undermining your IT department, give them the leader they need to succeed.
One final word, if your IT team is delivering value constantly they need to hear good things from you far more frequently than the complaints.