What mistake? Listen to the sound clip. The NBC "reporter" is talking about Muslims and data bases, and Trump is talking about walls and illegal immigration. I don't think Trump was even paying attention to the NBC guy fully. This is much ado about nothing. Just another failed attempt by the liberal media trying to create a "gotcha" moment.
Problem is, this is consistent with Trump’s previous positions on the surveillance state. That’s why it rings true. It is not inconsistent for Trump to support big government surveillance.
Donald Trump: Putin would return ‘total traitor’ Snowden if I’m president
http://www.freerepublic.com/focus/news/3327821/posts
[June 1, 2015 Video] Donald Trump on the need for NSA phone data collection
http://freerepublic.com/focus/f-news/3325865/posts
When you see or hear the phrase “Trump Refuses To Rule Out” from the media a blatant lie will follow.