Republican presidential candidate Donald Trump speaks at a campaign rally in Boca Raton, Fla., on March 13. (Erik S. Lessner/EPA)

Huge. Winning. Classy.

When Donald Trump finds a word or phrase he likes, he sticks with it. To wit: The 100-plus times Trump assured us that America is a laughingstock. Or the many times he has called someone a "friend of mine."

Over the weekend, he resurfaced another time-tested Trumpism: The often vague warning to us, to you, to them — to someone — to "be careful."

This time, he aimed those words at Democratic presidential candidate Bernie Sanders, who Trump accused of sending protesters to disrupt his rallies. It was at least the fourth time this month that he used some version of the phrase.

Below, we carefully rounded up 15 instances of Trump urging such caution to everyone from the Wall Street Journal editorial board to President Obama.

"We have to be very vigilant. We have to be very careful. And we can't allow people coming into this country who have this hatred of the United States." — March 10, to CNN's Anderson Cooper

"The Republicans are eating their own. They've got to be very careful." — March 5, at a Kansas rally

"He has to be very careful." — March 3, to CBS News's Major Garrett, about Sen. John McCain (R-Ariz.)

“Carly was a little nasty to me. Be careful, Carly. Be careful." — Aug. 14, at a New Hampshire rally