False belief: That in the U.S. the death penalty was cheaper than life in prison.

Believing this wasn't rational. I didn't take such basic steps as looking up the costs surrounding executions or life imprisonment. Executions get much more appeals, trials and legal attention.

False belief: That in the U.S. deaths by firearm are generally homicides, not suicides.

Believing this also wasn't rational. I didn't take such basic steps as looking up available death statistics.

Actually, looking through things potentially on the list for me, a lot of them seem to have the following general form:

1: Something is asserted.

2: I think: 'Yeah, that sounds plausible.'

3: I don't bother to look up any data about it, I just move myself to the believe column.

4: Later, someone else reports data about it.

5: I'm surprised that my earlier beliefs were wrong.

I've since became more skeptical of believing things based on just assertions, (I can even recall a recent instance where an assertion popped up on TV which my wife believed, but which I was skeptical of and which upon looking it up we found data didn't support it and that they were massively overstating their case)

But I can definitely recall beliefs that I have had in the past that were fundamentally just assertion based and the followed the above pattern.