One thing I’ve found to be useful is just having my browser clear all cookies upon closing. It’s initially annoying while you set up all your exceptions for commonly used sites so you don’t need to log in again there every time, but afterwards you don’t need to worry too much, because once you close your browser, all the useless cookies are gone.
It’s likely a difference of emotion compared to logic. Emotionally they’d think “Damn it, now we need to check for such a weird specific edge-case, this is so annoying” while logically knowing it’s better the tester caught it.