This incident occurred back in 2007 a 42 year old man in Canada went in for leg surgery. A totally routine procedure. But when the surgeons made the first incision, they noticed something... unusual. His blood isn't red. It's green. Like, deep emerald green. Everybody in the room pauses.
The team runs tests to see if there was a lab mistake or some sort of contamination, but nothing like that, it is real. The man had an extremely rare disease called sulfhemoglobinemia, where sulfur atoms bind to hemoglobin, that is the molecule that carries oxygen in the blood, changing its structure and the resultant color. And thus, his blood looked like some sort of sci-fi goo.
It turns out he had been taking sulfonamide drugs for a chronic infection, which are drugs with sulfur in them. After enough accumulation, his hemoglobin became chemically altered. The crazy part is that this specific type of hemoglobin does not carry oxygen properly, however if you don't have a lot of it, your body can sometimes manage.
There were no significant complications during the procedure, and once they discontinued the medications, his blood reverted back to normal over the course of a couple of weeks. The whole thing is crazy, there have only been a handful of cases published in medical journals where these white blood cells quenched all of his hemoglobin and he lost all of the original color of his blood.
Imagine being the doctor about to make an incision. You cut the skin and then you see something totally unexpected: green blood. I would have thought I was hallucinating.
References:
https://www.patientcareonline.com/view/patient-green-blood-doesnt-faze-surgical-team