To recreate, have Clive's Hideaway out, and simply plot any spell. In my test games I used Fblthp, Lost on the Range to plot from the top of the deck. Plot states it is in exile and you may cast it on a later turn at sorcery speed right? However, I found with the third ability on Breeches, Eager Pillager (Exile the top card of your library. You may play it this turn) , you can cheat out any plotted cards the turn they are plotted. Plotted before the Breeches trigger? They are stuck in exile for a turn like normal. Plotted ANY TURN after a Breeches trigger? They are all castable for free the turn they are plotted. I am not sure why but these are the 3 cards I found that can recreate this exploit.

TLDR: Have a Clive's Hideaway, plot any card, trigger Breeches, all plotted cards ignore turn restriction.

The attached pictures show 2 different games with 2 different spells that were plotted that turn, and are still able to cast immediately for free. I figured it has to do with Clive's Hideaway because they all say "exiled under" Clive's.