The Hulk has one of the most powerful healing factors in the Marvel Universe, but his regenerative powers have an incredibly dangerous flaw. Most other characters with healing factors like Wolverine and Deadpool will always heal back in the same way, as their bodies rapidly repair damage. However, if Hulk heals at the wrong time, he could totally destroy himself.
Bruce Banner and the Hulk have had a variety of intelligence levels throughout their shared history. While the default is usually that Banner is the genius and Hulk is a temperamental brute, the two have occasionally swapped minds, as well as manifesting different personas like Professor Hulk and Joe Fixit. Normally, the Hulk’s lack of intelligence doesn’t hamper him too much, as very little can harm him given his massive strength, god-tier durability and incredibly effective healing factor. In addition, Banner can transition into Hulk form very quickly when he is in danger, which makes him very tough to actually kill. But the Hulk’s enemies don’t actually have to kill him to neutralize the threat he poses.
In Hulk #3 written by Mark Waid and penciled by Mark Bagley, Bruce Banner is still suffering from the aftereffects of two gunshot wounds calculated to take him out of commission before he could protect himself by becoming the Hulk. While Bruce was able to prevent fatal injury, he suffered extensive brain damage that is affecting his cognition. Bruce is warned by Maria Hill that while his healing factor is fast, it's also inexact, effectively 'rushing' the healing process. This means that if he transforms, his brain will 'heal' in a way that only makes his injuries worse, as his brain reknits according to the pattern of its current damage. Sadly, when the Abomination strikes, Bruce must nevertheless become the Hulk to take him down.
Hill's guess is proved totally correct after the battle, as Bruce's cognition is harmed even further. Worse still, since his brain has been rebuilt, there's nothing left for his body to 'heal,' and no natural way to recover from the damage. Thankfully, Iron Man is able to use the genetic manipulator Extremis to rebuild Bruce's mind, but without a once-in-a-lifetime genius inventor on call, Bruce would have destroyed his memories and personality by transforming into the Hulk while brain-damaged.
It's a mark of Bruce's heroism that he took the risk anyway, but it also shows that Hulk isn't as invulnerable as he seems. Some superbeings - like X-Men's mutants - have powers which are naturally attuned to their bodies, but Hulk is the result of devastating radiation. It's no surprise that some of his powers can actually make things worse for him when they're simply the way his body reacted when bombarded with gamma energy.
Hulk is a chaotic hero whose strength comes at the cost of his self-control, and his healing factor's ability to heal him in the wrong way is the ultimate encapsulation of this. Not only are there injuries Hulk's healing can't actually fix, but in the case of Bruce Banner's most serious ailment, the healing factor made it both permanent and far worse. Bruce Banner has spent his life as the Hulk trying to manage his other self in order to prevent harm to other, however he also has to be wary of how turning into the Hulk could destroy his sense of self.