Qualified immunity doesn't apply to criminal cases. It is used to defend against civil suits. It is unfortunately very easy to find many cases where it leads to injustice. For example:

>...Abby Tiscareno, a licensed daycare provider in Utah, was wrongfully convicted of felony child abuse when a child under her care suffered brain hemorrhaging. After calling emergency services, subsequent medical tests supported these findings. However, during her trial, requested medical records from the Utah Division of Child and Family Services (DCFS) were not provided. It wasn’t until a civil suit that Ms. Tiscareno saw pathology reports suggesting the injury could have occurred outside of her care. She was granted a new trial and acquitted. Her subsequent lawsuit for due process violations, alleging that DCFS failed to provide exculpatory evidence, was dismissed due to lack of precedent indicating DCFS’s obligation to produce such evidence.

https://innocenceproject.org/news/what-you-need-to-know-abou...