What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.
What about the methodology of starting with news reports? These have strong biases that probably change by time. And how did they locate decades old news reports? A recency bias is to be expected. (Added: Duwe has a paper about this!)
Their definition is pretty reasonable, except for the part where they make tons of exceptions. The examples they gave of the exceptions that they made sound intuitive, but what about all the exceptions that they didn't talk about? Why didn't they include the Ridgewood Postal murders? Since they don't actually have a consistent rule, it's impossible to decide if they applied it to this case, or if the actual criterion was that it was too old.
Added: I tried spot-checking a few examples from Duwe's book against the MJ list. On pages 115-116 he lists 12 high profile workplace mass shootings. In addition to Ridgewood, they omit Alan Winterbourne and Willie Woods, both very straight-forward examples. If they miss these high profile examples, why would you expect them to reliably find others?
Thinking about this more, I think you are correct. The data is much too spotty to make a strong conclusion.