The Flaw in Flawless
There's one major problem with flawless characters: They're boring. They don't screw up, so there's no tension in watching them. For them, there's no trying, there's only accomplishing.
While there's a certain probability that the good guys win, you never know how often they stumble on their way. That makes watching characters' journeys (literal or metaphorical) worth following. Working around flaws, having them exploited and learning from the experience... all that is character growth. No flaws mean no learning, no growth and no change, and that ultimately makes a character less deep than you'd want them to be.
When is a Flaw a Flaw?
We already established that flawless characters are kind of... meh. That's why the writers of these characters are often told to add flaws to make their character less flat. This is when pseudo-flaws come in.
Rule of thumb: If a flaw doesn't bring your character in trouble/cause inconvienience/isn't treated as one in-story, it doesn't count.
This means that many things that sound like flaws aren't, but on the other hand, everything can be turned into a flaw. I'll just throw in a list with my favorites.
This one is too easy. Just have your character stumble a few times and hah, a flawed character.
Not a flaw, because... this mostly serves to make the character endearing. Also, they can stumble in their one true love's arms and deer-eye them.
Make it a flaw: Turning clumsiness into a flaw is really easy. Have the character stumble while carrying something important. Have people be angry at them for spilling something. Clumsy kids aren't considered endearing and cute by their peers, they're being laughed at.
Where do I even start? "Oh no, I'm so beautiful it's a curse!" Ergh.
Not a flaw, because... no matter how much the character whines about being too beautiful, nothing harmful ever comes of that.
Make it a flaw: Beauty is superficial and thus attracts superficial people. It's coupled with expectations in both men and women. There's jealousy from others. I still wouldn't call this the main flaw of any character.
Yes, that exists, too.
Not a flaw, because... being helpful is kind of a good thing. And these "too helpful" characters are often just a decent amount of helpful, not really too.
Make it a flaw: Where there are helpful people, there are also jerks who abuse that. But it's not just others that can cause trouble for the overly helpful character. If they really want to help everyone, they can easily overburden themselves or feel guilty for failing people in need.
I could continue this list for a while, but the bottom line is that many pseudo-flaws can be turned into real flaws by adding consequences.
Too Many Flaws
Flaws don't replace believable writing. If you just pile up flaws on your character, you'll risk making them unrelatable the same way you do when you add none at all. Also, failing at everything, if it's not in a slapstick comedy, isn't all that entertaining.
So, as with everything, try to find a middle ground between squeaky clean perfect and oh god how can this guy even survive a day. And, to quote a Mary Sue Test on this, if you ever describe your character as too perfect, douse yourself in cold water now.