I think mistakes and errors in research do often result in good conclusions. Isn't that how penicillin was found?
Mistakes do happen, but then they use them if possible. So in this case, rather than discarding that group, they decided to look at them as a separate group and study that. They've been honest and upfront about it, so it doesn't concern me at all.
If they start hiding the mistakes-which they could have done, then that's when you worry.
I would be more worried about the US one simply because over there there is so much financial pressure to be the first that they can be tempted to skip steps or cover up mistakes rather than admitting them.