IF YOU WATCHED Sunday's debate, you may have gotten the feeling at times that President Reagan and Walter Mondale have been living in different worlds. Take, for example, their exchanges on Social Security and Medicare.

Mr. Mondale said that the president, in his first year in office, proposed "to cut Social Security by 25 percent. It's not an opinion -- it's a fact." The president replied that, "the only 25 percent cut that I know of was accompanying that huge 1977 tax increase was a cut of 25 percent in the benefits for every American who was born after 1916." Which actually happened in the world in which most of us seem to live?

Mr. Mondale is quite right that the Reagan administration proposed -- and in a few cases got -- substantial cuts in Social Security benefits affecting both current and future beneficiaries. A first set of cuts -- affecting widows with college-age children, recipients of minimum benefits and beneficiaries of lump-sum death benefits -- sailed through Congress. Congress had second thoughts about taking away money from thousands of octogenarians, and later restored minimum benefits for current recipients.

Perhaps emboldened by its success, the administration unveiled a far more ambitious plan that would, all by itself, have cut future Social Security benefits an average of 25 percent. The projected savings, moreover, were substantially larger than needed to ensure Social Security solvency. These proposals were so unpopular with both Congress and the public that the administration hastily withdrew them -- perhaps that explains why they seem to have dropped off Mr. Reagan's screen. As he recalls, the president then appointed a bipartisan commission, which eventually came up with a solvency plan. But he doesn't seem to remember that the commission plan almost died because, despite strong urging by Senate Republican leaders, he wouldn't go along with it until the last moment.

What about the 1977 benefit cut that the president mentioned? No doubt he is referring to the fact that the bipartisan plan enacted in 1977 not only raised payroll taxes, but also corrected a glitch in the formula used to adjust initial benefits for past inflation and wage growth. Under certain economic scenarios, future retirees might have gotten benefits 25 percent larger than if the error hadn't been corrected. It's not what you would normally call a benefit cut, but it certainly saved money for the trust funds.

With respect to Medicare, Mr. Mondale said, that having promised not to, the president "went out and tried to cut $20 billion out of Medicare." The president flatly denied it. In fact, Medicare was left almost untouched in the administration's 1981 budget cuts. Later the administration proposed various Medicare cost restraints -- including a hospital cost-control plan not all that different from the Carter administration's.

Congress didn't give the president everything he wanted, but it came up with ideas of its own so that Medicare savings are projected at several -- though not as much as 20 -- billion dollars over a few years. It's worth noting that most of these savings are supposed to come out of the pockets of hospitals and doctors rather than those of patients. And Mr. Mondale ought to remind himself that, should he become president, he'll be mighty glad the Reagan administration took some steps to rein in Medicare's runaway growth.