"I forgot to buy papers."
I believe it's because 了 after the verb implies a duration. For example, 去了 and 寫了 are acceptable because movement and writing have defined beginnings and endings. 忘記, on the other hand, is of an either-or duration (you either forgot to buy paper, or you didn't). [If you wanted to say something like "I forgot to call him for two days." you'd have to add a similar duration expression into the Chinese translation.]