Generally speaking, superheroes are smart people. They have to be otherwise they wouldn't survive doing what they do for long. Oliver Queen and his friends and allies have been through many challenges and they managed to defeat most of them.
That doesn't mean they always make the right choices. Quite the opposite, in fact. as the characters evolved over the course of Arrow, they became more careful. Even then, though, they kept making mistakes. Not just in their personal and work life, but also when it came to choosing their outfits. It might not seem like a big deal but some Arrow outfits were truly impractical and endangered those who wore them.
Oliver Queen's Initial Outfit On Lian Yu
These clothes were just all wrong. To be fair to Oliver, it wasn't his fault - he simply wore this when the ship crashed. However, he made zero effort to mask himself in any way.
The white shirt he wore on Lian Yu shone like a beacon among the greenery of the island, which meant Oliver drew a lot of attention to himself. And considering how dangerous Lian Yu is, that could have easily proved fatal for Oliver. Luckily, he later switched these clothes for some tactical gear.
Oliver's First Arrow Suit
Speaking of Oliver and Lian Yu, there's also his first prototype of the Green Arrow costume he wore when he crashed on the island.
It looked like a mixture of two different outfits and it had none of the useful traits or resilience of his subsequent suits. At least it seemed it allowed Oliver to move freely, but that's about it.
Leather Jackets Everywhere
Maybe it's because of budgetary issues, but a lot of Arrowverse heroes wear leather jackets as the base of their costumes. Leather jackets do look good but they, once again, provide very little protection against harm.
Also, depending on the exact material used for the jacket, leather can restrict movement, which is something superheroes can't be grateful for in combat.
Thea Queen's Training Outfit
In the third season, Thea lives with her father Malcolm, who teaches her how to fight. In one of the scenes, Thea wears a helmet typical for the martial art Kendo.
Except she doesn't have it properly tied, since she easily takes it off her head once she stops fighting. In real life, if she fought in this helmet and wouldn't tie it, it wouldn't hold the position on her head and make fighting very difficult, if not impossible.
Rene Ramirez's Upgraded Suit
Rene Ramirez AKA the Wild Dog has an iconic look in the comics, topped by his hockey mask. He sported this look on the show as well - until the creators decided to replace it with an upgrade that, frankly, didn't make much sense.
Rene's whole spiel is that he's a street hero who moves around fast. The clunky upgraded suit looked like it would considerably slow him down, which is the last thing Rene needs. So it was a good thing Arrow eventually returned back to Rene's old suit.
John Diggle's Mask
When John Diggle finally got a mask that would protect his identity... it was a disappointment. Not only does it look weirdly reminiscent of Magneto's helmet, but it also doesn't really hide John's face features that well.
Plus, it looks like it would be heavy and uncomfortable to wear on the head for a prolonged period of time. What's more, it significantly limits John's field of vision, which is a dangerous thing for any fighter.
Ragman's Rags (Without Its Power)
At one point, Rory Reagan's Ragman costume lost its power. It no longer shielded him from harm or helped him bring down his opponents.
When Rory realized this, he temporarily left the team since, without its power, his Ragman costume was... well, rags. Luckily, the costume later regained its power and Rory helped the team out in the future.
Open Arms Costume
Oliver sported many different suits on Arrow. One of the less practical ones had him show part of his arms. That might not seem like such a big deal at first, but by uncovering a significant part of his arms, Oliver also opens up himself more to possible injury.
There's nothing - no cloth, no obstacle - protecting his arms, which makes it that much easier for his opponents to slash at him, cut him, or hurt his arms in a different way.
No Mask For The Hero
It's true that even masks sometimes don't cover enough of the face to make a difference. But Oliver took this one step further and he put himself at unnecessary risk.
In the initial seasons of Arrow, he wore no mask at all. Instead, he used a bit of green paint that he smeared around his eyes. If somebody managed to snap a photo of him or got a good look at his face, no amount of green paint would help Oliver to protect his identity.
Every Outfit With A Hood
This concerns not only Green Arrow himself but also other superheroes, such as Arsenal or Speedy. Anybody who has ever worn a hood will attest that it significantly limits what the person sees. That can be dangerous, especially if the hero is outnumbered and surrounded from all sides by their enemies.
In other words, hoods are helpful for concealing the hero's identity but they can also endanger them.