Mistakes, I’ve made a few, but then again, too many to mention. I like to think I’m a pretty good tester, and that didn’t happen without making a LOT of mistakes. I suspect that most were the result of making assumptions that turned out to be wrong. Plenty were the result of trusting “common knowledge” instead of checking authoritative sources.
The ones with the worst outcomes were probably unavoidable because they involved factors I didn’t know about. I would argue I couldn’t realistically have known about them, but that’s of little consequence when a company is looking for a scapegoat. I have written about some of those mistakes in posts at:
Epic software bugs - major business losses and heavy user impact - #3 by steve.green,
Epic software bugs - major business losses and heavy user impact - #4 by steve.green
Your failures, not failures you've prevented? - #4 by steve.green
I only lost confidence once, very early in my career around 2003 when I started going to conferences and realised I wasn’t doing testing like anyone else. From 1995, I had been self-taught and was doing what I later found out was exploratory testing. The little training I received from ST Labs in 1999 reinforced that. But at the conferences, it was all ISEB/ISTQB, IEE829, test scripts, test documentation, UML, model-driven testing etc. Surely they couldn’t all be doing it wrong, so it’s got to be me?
Fortunately, I met Alan Richardson at a SIGiST conference and he introduced me to James Lyndsay and James Bach, who helped me realise that everyone else was indeed doing it wrong.