See, I am right!
Bias is a disproportionate weight in favor of or against an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group, or a belief.1)
There are many types of bias and everyone, including serious and intelligent scientists, are guilty of it.
Take for example what is called 'confirmation bias'. This is the tendency to take seriously that which confirms your belief rather than that which would refute your belief. Someone who is hypochondriac will find 'evidence' all over the Internet that, upon seeing a spot on their arm, they definitely have cancer. The article or the doctor, who says it is a harmless pigmentation is not believed. The hypochondriac is already convinced and will therefore look for and find confirmation everywhere to back up his conviction. Anything that doesn't support his belief is overlooked or denied.
But beware: not only hypochondriacs do this. Everyone has more or less this tendency. Social media have made this bias almost standard by programmatically showing more of what matches a previous search of the user. And so after seeing an innocent video about the "face on Mars" you also end up with the apparent image of Jesus in a piece of toast and from there to other "paranormal" states and before you know it you see nothing but abductions by aliens and ghostly apparitions.
Seeing a face on Mars or Jesus in a piece of toast is another form of bias, called apophenia or pareidolia: the tendency to see patterns in what are actually random configurations. And everyone knows the 'face' on the moon.
People naturally tend to see faces in everything, in the dots of the balatum lino on the floor or in stumps of trees. I guess it has to do with survival as a baby to recognize the mother's face. And the example of seeing a snake that is in fact a piece of rope is also familiar to everyone. Recognizing danger in time is of course of the utmost importance, but it can also be very misleading.
I think it's a good idea to be as aware as possible of these kinds of sneaky constructions that cause you to think "See," "I'm right (after all)." You can find an overview of all kinds of bias in the Wikipedia article that talks about that.