That tends to be how it goes, unfortunately. Despite how bad some PuG tales are, you usually zone in expecting things to go all right. So you might not notice some things right away unless it's noticeably impacting your experience. Heck, sometimes I've forgotten to put on Shield Oath due to the level adjustment but I'm spamming my enmity moves enough that I'm maintaining aggro so I don't notice it until someone tells me a few pulls in.
That's just a low key example of it, but stuff like healers ONLY healing or DPS being problematic (such as continuing to single-DPS in big groups or, almost at the opposite end, spamming AoE stuff with single and two-mob pulls) aren't always noticeable right away. Or DPS/Healers in bad or broken gear - it's far more noticeable on tanks since they'll be horrendously squishy (though that could also be assumed to them just not using their cooldowns). I've had experiences with all of them in some shape or form, and I usually don't really notice it right away. Too wrapped up in my own role a lot of the time.
That's just a low key example of it, but stuff like healers ONLY healing or DPS being problematic (such as continuing to single-DPS in big groups or, almost at the opposite end, spamming AoE stuff with single and two-mob pulls) aren't always noticeable right away. Or DPS/Healers in bad or broken gear - it's far more noticeable on tanks since they'll be horrendously squishy (though that could also be assumed to them just not using their cooldowns). I've had experiences with all of them in some shape or form, and I usually don't really notice it right away. Too wrapped up in my own role a lot of the time.