If I flip a coin one million times and only publish the ~500,000 results that show coin toss coming up as "heads," I could lead people to believe that a fair coin will always land on heads.
That's how misleading this bias can be.
But anyone can toss a fair coin, so I'd be found out pretty quick. But with more complicated and murky stuff like psychology this can be a huge problem, especially if people begin using this shaky work as foundation for things like: public policy, how to live their lives, how to do work, etc.
After all they can't just toss a coin to check my work on the more complicated stuff.