The Green Lanterns had the dumbest weaknesses of all time. Almost every superhero has a weakness (or weaknesses) of some sort, either external or internal. Even DC Comics' Superman can be harmed and killed with kryptonite (irradiated pieces of his homeworld). Even so, some weaknesses are just too ridiculous to persist throughout their characters' histories, as was the case with the Green Lanterns. But the Green Lanterns actually had two incredibly dumb weaknesses in need of retconning: the color yellow and... wood (seriously).
In DC Comics, Green Lanterns are extremely powerful officers of the cosmos, led by the Guardians of Oa. Wielding power rings harnessing the green light of willpower, the Green Lantern Corps work to keep the galaxy safe, having broken up the universe into a collection of sectors with assigned officers to each one. However, one of their original weaknesses was their rings' inability to affect anything that was the color yellow, the color of fear. However, this meant that something could simply be turned yellow and the Lanterns' rings would have no effect on it whatsoever. As a perfect example, Batman once painted an entire room, Robin, and himself yellow in order to taunt Green Lantern Hal Jordan, all while drinking a glass of lemonade in All Star Batman and Robin #9.
However, this is nothing compared to the weakness of Alan Scott, the first Green Lantern, though his powers are more magical in nature than the members of the Corps. In one of his first stories, Scott was caught by surprise when he was hit in the back of the head with a wooden club, which motivated him to believe that his ring's power had no effect on wood. In reality, the ring makes the thoughts of its wearer real, which meant that Scott subconsciously created his own weakness to wood going forward, when the truth was that he was simply caught off guard. Thankfully, both of these extremely dumb weaknesses were retconned and corrected in later years.
Alan Scott's retcon was actually quite simple: he realized his weakness to wood was a figment of his own mind and the weakness was eliminated. However, fixing the Green Lantern Corps' weakness to yellow was a little more complex. During the Green Lantern's Rebirth era under Geoff Johns, it was revealed that the "Yellow Impurity" was the result of the fear entity known as Parallax being trapped within the Central Power Battery, the rings' source of power. Once Parallax was removed, the Impurity was corrected and the rings could then affect yellow objects and foes. This was good timing too, seeing as how the formation of the Sinestro Corps was just around the corner, wielding rings that harness the yellow light of fear.
Clearly, eliminating these weaknesses had to happen, and they were just too ridiculous that it made it quite hard to take the Green Lanterns seriously. Furthermore, it's not as if they're invincible now that these weaknesses are gone. The Lanterns still have to recharge their rings when their power is depleted, and there's plenty of threats in the DC Universe to still pose a significant challenge even to the strongest Lanterns. Furthermore, the aforementioned Central Power Battery just exploded in recent issues of Green Lantern, so it certainly seems as though the Corps have their hands full even with their dumb weakness to a color being corrected.