One of mine is when someone looking for a healer and says “we have a strong group”. Shows overconfidence.
2 others are someone too over levelled or too under levelled for a quest or dungeon. It absolutely could be genuine, but I’m not going risk my one life to find out.
What are yours?
I think it’s common to be overlevelled for dungeons at the start of HC. For example, ranged can go DM at 20, 22 for melee.
I’ve seen players attempt at lower levels though in an overall low level group and well…
From Era:
“T3 geared tank”
It almost always turns out that yes, he indeed is T3 geared, but he is arms and don’t know how to tank at all.
Nothing wrong in being overleveled. It’s safer that way. I had couple sketchy situations, and if I were 2-3 levels lowers - probably I would’ve died.
Red Flag:
Get invited and no body talks, even when you ask them.
Communication is the key to success in a grp.
Intend to exploit, like in Uldaman.
Exploiter are bad fighters.
I had a level 30 hunter help me with a quest, I was a level 23 hunter myself. They proceed to pull lots and feign death by me.
Being overlevelled can mean they’re bored and looking to entertain themselves by other means.
Nothing wrong with it sure. But it can be a red flag that a helping hand isn’t always as it seems.
People who complain that you are not in the tank/healing spec for some low level dungeon and write walls of texts. Those are the first to dip when you pull one extra mob
Hunters who use Multi-Shot even if there is only one or two mobs.
Shadow Priests.
I’m sorry but yall generate too much damn threat, it’s very hard to deal with.
Group has 2 of any hunters or rogues. I expect them overpull and bail out leaving party to die sooner than later.
DPS posing as a healer. A competent player can pull this out in WC/DM but starting from BFD I do not trust none healer to not have aggro or mana issues.
Solo mage urging tank to do bigger pulls. No little fella, unless there is second mage to cover your mistakes and occasional mob resists I do not trust you to handle AoE situation, especially if there are ranged and healer mobs in the pack.
Constant rushing, negation of clearing of patrols and general lack of CC on mobs with nasty spells or heals. Like going into that golem boss room without clearing trash in all passages around it first in Uldaman.
And my favourite one. Tank rushing towards Mograine in SM Cath without clearing left and right alcoves. 
i instantly leave group if i notice somebody uses the term “red flag”. f’in zoomers.
Questing near a mage that AoE farms and hunters in general.
And a hunter dps in a dungeon group is a pass for me, I have seen too many hunter moments happen in my 15 years of wow career.
You can heal without being a healing spec all the way up to BRD with no problems though. Imagine requiring someone to be heal spec in BFD, a level 20 + instance :))
You sir, are a 
and a red flag.
While this is true, there is also the big difference that a dps-specced healer most likely will not be able to save the group when the hunter does his “Oops” pull.
“Inv to new layer Plzzzz!!!”
There is literally zero difference. The 10% more healing a healing spec does, does not justify spending 100g on two respecs just so mr warrior can have a healer with healer spec. I doubt you spec into deep prot either.
Oops pulls are team effort anyway.
Already had a classic andy warrior demanding me to spec holy for zul farrak and then leaving the group after writing walls of text. Nevertheless we got another tank and had absolutely zero issues.
I healed as a druid in 2019. When my resto was lvl 39 I healed a lvl 40 tank in Uldaman, who missed all his tants on lvl 47 last boss, while dps were lvl 45-47. We lured boss upstairs, but feral bearly 
could manage aggro on boss, not mentioning adds. So we desided to let a rogue off tank adds and I held both rogue and feral alive. That worked and we finished the instance. This is what properly talented healer can do. Mana efficiency, extra instant casts in case of emergency, less aggro issues in massive damage taken by the party situations. Invaluable things if you have no room for mistakes.
You are free to take any dps talented player, posing as healer, but do not ask why he didn’t had mana allready mid combat or had ripped aggro from a tank in massive damage intake situation and died, dooming your group.
Nobody talking about respecs. You ether lvl up with a friend or do not have dedicated healing spec and risk wiping every instance because of your lack of that edge healing talents give for your role, because you can’t level exclusively in instances only unless you want to level up in 6 months
You have 0 knowledge of the game if you think a healing spec is what makes the difference in ANY content below BRD and maybe ST. You must be delusional and in actuality, players like you are the real red flag. Crying about healing specs while wielding a 2h as arms and taking on half the instance as a “tank” is good, but god forbid the healer has a different spec. You absolute
Please cry me a river next time dps with some int and spirit gear (because obviously he did rolled on dps gear whenever possible and has little to no spd gear) won’t hold that crucial 10% of your HP sending you back to Dun Morogh.
Is it possible to heal in Classic dungeons with dps spec? Yes. Will you trust someone without dedicated healing spec to withstand the extreme situation that may happen in PUG? I know I won’t, thus red flag. I will rather go solo leveling, than trust some shadow priest or retribution paladin doing the healer duty.