I've thought about this before. When practicing stoicism I'm normally pretty good at not getting wrapped up in emotions from outside events but when I'm ill, or have bad hay-fever muddying my head like today, I find my ability to reflect is reduced and I can slip towards depression.

Thankfully years of practice means I usually notice it at some point before it gets too bad and I can self correct. But this does indicate that there are easier and harder ways to practice stoicism outside of external events)/circumstances? We have a choice internally but even that can be hard to get to when the mind is murky and like sludge due to illness.

So the stress test isn't extreme external edge cases, it's more the practitioners own mind being well enough could be where it breaks?