Warning! Spoilers for Tony Stark: Iron Man by Marvel Comics below.
In his efforts to save Janet Van Dyne from an Ultron-Hank Pym hybrid, Iron Man decided to wear his Ultronbuster armor, which was specifically designed to counter the villain. However, Ultron-Pym's experiments on molecular fusion and anger at Janet and Tony Stark's romance led him to fuse the hero with his armor, creating one of the most horrifying versions of Iron Man ever.
The unfortunate part of being a mad scientist with billions of dollars and a thirst to push the limits of man-made power is it can lead to some horrifying inventions and situations. Iron Man has had his armor try to kill him, has accidentally turned it into a living creature, and even created armor that came from his skin. Tony doesn't seem to have a moral compass when it comes to pushing the envelope on what his armor can be. However, one of his most horrifying experiences wasn't a result of his own tinkering. Instead, his body bonded with his armor when he tried to stop Ultron-Pym.
In Tony Stark: Iron Man by Dan Slott, Jim Zub, Christos Gage, Francesco Manna, Edgar Delgado, and VC's Joe Caramagna, the story dealt with Iron Man trying to stop a hybrid version of Ultron/Pym from turning Jacosta and Janet Van Dyne into one single being. However, when he came face-to-face with the villain, he was shot with a blast that fused him into a monster. The results of the fusion are disgusting, as Stark's body became a messy mix of technological features from his armor and his body.
While Stark's armor was an abomination on the outside, it also started to slowly kill him from the inside - forcing Tony to constantly reprogram his armor to keep his internal organs from changing. The constant reconfiguring kept him alive against Ultron-Pym. Thankfully for Stark, the fusion was reversible, as he used a nano-bot to go inside his own body alongside Janet and removed the technology inside.
In this case, Stark's Ultronbuster armor fusion wasn't his fault, as Ultron-Pym used his own technology to turn the hero into a monster. Among the most creepy and messed-up armors Iron Man has found himself being forced to wear, few of them top the disgusting nature of his merged armor. In the end, Stark got his body back by pulling a Ms. Frizzle and would defeat Ultron-Pym, but there's no question his Ultronbuster armor fusing with his body almost cost him his life. Iron Man was lucky to make it out of the ordeal unscarred.