It was clearly a mental mistake. You can clearly see the pace of the game was slower than Spurs usually play, and Pop expected the Heat to watch the tape and adjust according to the pace and the way the Spurs played in Game 1. Because of this, Pop had the idea of pacing the offense, making it even faster in Game 2 to run the big 3 out of the building by putting the bench players out to make the pace really fast. In the end, it just bit Pop in the ass because we ended up making a ton of turnovers with that pace and the Heat made the most of it.
It all boils down to the same conclusion: that's why teams like the Phoenix Suns and Golden State Warriors never make it into the finals... that fast paced play will never fly in the finals. You need to be really gritty on defense and take your time on each possession in order to make sure your teammates are there to box out for the rebound or able to get back on defense on time.
I expect the pace of game 3 to be just as fast, if not, slower than game 1.