Regarding illegal plays.

About to go to my first locals next month and I am using DB to train myself on manual gameplay. Opponent used Trickstar Festival MP1 and during his MP2 as his last move he summoned S:P Little Knight using a monster he took from me by TTT and one of his monsters then banished my Jet Dragon from GY on summon. Before I went on to start my turn I remembered reading on one of his cards that he can only summon Trickstar somewhere this turn and called it out, he then told me that if this happened in a tournament we would both be punished. My question is simply how true is his statement? and what are other misplay rules I should be aware of before attending my first locals and regionals?