Because Carbon Dioxide Looks Entirely Different.
17 years ago
General
From Station, a comic book about a murder on the International Space Station, written by Johanna Stokes, who apparently also writes for the Sci-Fi Channel's Eureka. A character finds a hypodermic syringe, and glances at the contents, saying:
"Looks like nitrogen."
Pat yourself on the back, Johanna.
"Looks like nitrogen."
Pat yourself on the back, Johanna.
FA+

I think my brain just broke a little.
Although, it's vaguely reminiscent of... "Iocane powder. I'd bet my life on it."
And since the syringe was supposedly used to inject water into a pressurized nitrogen tank, how did nitrogen end up in the syringe?
Sorry; this is just some of the worst reasearch I've ever seen in a published work. A friend of mine spotted seven outright errors and three glitches on one page.
The script reads: "And then the astronaut holds a syringe filled with nitrogen."
The artist asks: "What does nitrogen look like?"
The author says: "...Oh."