Another one was GPS: They had prepared two sets of maths for the satellites, Newtonian and relativistic. They started operating them with the Newtonian model, and the satellites went out of sync, nothing really worked. Then they flipped the switch to relativistic, and everything worked flawlessly.

Even before that they took an atomic clock, put it on a plane, and flew it around the earth to later compare to one that stayed on earth. They differed by the expected fraction of a fraction of a millisecond.

Neither of those two could be done right when Einstein proposed relativity, but experiments like that could already be envisioned, "move a sufficiently precise clock sufficiently fast and compare it to a stationary one" is kind of a no-brainer. That's not the case with string theory, noone has any idea how to test any of it.

OTOH, physics shouldn't feel bad about that stuff. E.g. number theory is notorious for results which are considered useless even by the people formulating them, only for an application to appear a century or two later.

Source?