Few college pranks can be said to be more grandly conceived, carefully planned, flawlessly executed, and publicly dramatic

Wiki:

The Great Rose Bowl Hoax was a 1961 prank at the Rose Bowl, an annual American college football game. That year, the Washington Huskies were pitted against the Minnesota Golden Gophers. At halftime, the Huskies led 17 to 0, and their cheerleaders took the field to lead the attendees in the stands in a card stunt, a routine involving flip-cards depicting various images for the audience to raise. However, a number of students from the California Institute of Technology managed to alter the card stunt shown during the halftime break, culminating in the display of the word "CALTECH," a common nickname for the Institute.
The prank received national attention, as the game was broadcast to an estimated 30 million viewers across the United States by NBC. One author wrote, "Few college pranks can be said to be more grandly conceived, carefully planned, flawlessly executed, and publicly dramatic" than the Great Rose Bowl Hoax.
The hoax was planned by a group of Caltech students, subsequently known as the "Fiendish Fourteen," in December 1960. They felt that their college, whose teams often played in Rose Bowl Stadium a few miles from campus, was ignored up to and during the Rose Bowl Game. The students decided to use Washington's flip-card show to garner some attention.
To discover the details behind the Huskies' show, a Caltech student disguised himself as a reporter for a local Los Angeles high school, and asked Washington's head cheerleader. They learned that, by changing the 2,232 instruction sheets, they would be able to trick unsuspecting Washington fans into holding up the incorrect signs.
The students broke into the hotel where the Washington cheerleaders were staying, and removed a single instruction sheet from a bedroom. They printed copies and altered each page by hand. On New Year's Eve, three of the "Fiendish Fourteen" reentered the cheerleaders' hotel, and replaced the stack of old sheets with the new.

At halftime on January 2, the Washington card stunt was executed as the Caltech students had hoped. NBC cameras panned to the section raising the flip-cards as they uneventfully displayed the first eleven designs.
The twelfth design modified the design of a husky into that of a beaver (Caltech's mascot) but was subtle enough that the audience did not notice.
The thirteenth design, which called for the depiction of the word "Washington" in script to gradually appear from left to right (starting with the capital "W"), ran backwards (with the small letter "n" appearing first). Other sources say that the routine intended to spell out, "HUSKIES," but that it had been altered to spell out "SEIKSUH." Regardless, it was dismissed as a simple mistake.
The fourteenth design, however, was an unmistakable prank. "CALTECH" was displayed in big block letters on a white background.
Mel Allen and Chick Hearn covered the game for an NBC national telecast. The announcers and the stadium fell silent for several moments, only to subsequently break into laughter. As the Washington band marched off the field, the cheerleaders did not give the signal for the fifteenth and final image. The Huskies were unaware that the Caltech students had not altered the last design of an American flag.