Writing textbooks has got to be pretty tedious work. So you can hardly blame the writers when they slip in something that seems a little bit… off. My theory is that one of three things happens:

#1. The writer slips something in to see if anybody notices.

A word chart that says "OMG WTF STFU PWN3D"
Best optometry chart ever.

A word problem with the heading: "When am I ever going to use this?"
The heading asks a very good question that the problem doesn’t really address.
A picture of a family posing with somebody in a Spongebob Squarepants suit. Caption: "Here is an American nuclear family comprised of mother, father, and two children. Please note that the large yellow kid with the poor complexion is not a member of this nuclear family."
Just don’t tell Spongebob he’s not a member. He’ll be crushed.
"This chapter might have been called 'Introduction,' but nobody reads the introduction and we wanted you to read this. We feel safe admitting this here, in the footnote, because nobody reads footnotes either." Whoever wrote this is my hero.
“This chapter might have been called ‘Introduction,’ but nobody reads the introduction and we wanted you to read this. We feel safe admitting this here, in the footnote, because nobody reads footnotes either.” Whoever wrote this is my hero.
Crying: (def) what you feel like doing after writing statistics textbooks.
This explains every other entry on this post.

#2. The writer is so focused on the topic at hand that he/she forgets common sense.

A cartoon turtle says, "Should you say 'Nine and five is thirteen" or "Nine and five are thirteen"?
Hey, it’s a grammar textbook, okay? Cut them some slack.
Consider hybridisation in the following way. The mixing of a unicorn with a dragon leads to a hybrid, the rhinocerous!
Apparently chemists believe that rhinos were created by dragons and unicorns mating.
Drinking while intoxicated (DWI)
That’s for when you’re way, way past the point you should stop.
A pie chart showing safe uses of radioactive materials. Nuclear weapons is one of the slices.
So… uh… what exactly are the people who’re using it unsafely doing?

#3. The writer blacks out and temporarily loses his/her mind.

A picture of a cat wearing a parachute, labeled "Well prepared cat."
As if there’s any other kind.
Some really awkward stick figure drawings.
This is how you know it’s time to hire a graphic designer.
This exercise in synonyms is making me really uncomfortable.
This exercise in synonyms is making me really uncomfortable.
I guess that's one way of interpreting that picture...
I guess that’s one way of interpreting that picture…
A creepy statistics problems about the probability of a marriage falling apart.
I can’t help but feel like we’re getting a pretty dark look into this statistics professor’s personal life.
A man has a conversation with his son about what it means to be drunk.
Ok, this one is just hilarious.

These images were found on the following sites:

http://thankstextbooks.tumblr.com/

http://theberry.com/2013/01/15/goofy-things-found-in-school-textbooks-14-photos/

http://www.techeblog.com/index.php/tech-gadget/12-funny-yet-real-things-spotted-in-textbooks

http://www.huffingtonpost.com/2012/04/09/21-hilarious-textbook-fails-photos_n_1408451.html#s848311&title=Is_This_A