I am not saying that only private forensic science labs are prone to error, but . . .
There are more than 10 000 cases now under question due to either poor work or possible criminal behaviour in two companies contracted for this work. These mainly involve drug and driving-under-the-influence cases but also affect child custody cases. Some cases have had to be dropped because the original samples are either no longer exist or have degraded.
Once again the privatisation of a once national service has ended up causing injustice and extra expense. Add the profit motive into the equation and standards, especially expensive quality control (of both staff and methodology), can fall below acceptable standards.
https://www.theguardian.com/uk-news/2017/nov/21/forensics-data-manipulation-may-have-affected-10000-cases