Have you ever worked in an environment where there was an overriding sense of fear behind decision making and team behaviors?
Do you ever notice people are very quiet in meetings when you know they disagree?
That's what we had for a long time.
I still recall a Project Executive standing in the middle of the open team space and yelling "I'm going to fire you all and start again" (He also had other gems like "Conflict breeds performance". But I digress... ). The main point was that the teams were driven by fear.
Ironically - so was the Exec. It was his butt on the line if the project failed again.
No one likes to fail and get thrown under the bus, and it had been going on for 4 years. I even had developers that were too scared to touch their keyboards.
Agile teams should make mistakes. They should be allowed to fail and learn. In fact there is a general motto of "Fail fast". The emphasis should be on how fast they get back up, dust themselves off, learn from the mistake and re-adjust. If they never fail, they're probably sand-bagging.
Does your work environment allow for failure?
An example I use is that of a tightrope walker. If you want someone to learn to get to the other side fast, the best thing to do is install a safety net.
Otherwise they won't even take the first step.
Finding defects is part and parcel of software development. You can slam the development and testing team for letting one escape, or you can set-up an environment and behaviors that catch them as soon as possible. Test Driven Development. Automated Tests. Continuous integration. Continuous Delivery.
Fail Fast. Learn fast. It's the best way to succeed.