I have been seeing some posts on Facebook recently with Piccolo yelling at Gohan to dodge. I do not understand the reference behind it.
What incident is being referred to?
It comes from the Team Four star parody of Dragon-Ball, and it occurs during the part where Piccolo trains Gohan to fight the Sayans
One of the things Piccolo does is tells Gohan to dodge, and then hits him. It's a running gag throughout the series.
You can watch the parody at http://teamfourstar.com/
In it, Picollo is teaching Gohan to dodge. However, he ends up conditioned to freeze in fear of the word instead. This becomes one of the most popular running gags in the series.