What is incorrect about this sentence: The boy running past the library too quickly.
user
Let’s see:  “the boy running past the library too quickly” means the boy was running by the library and also that he was running too quickly.
assistant
What is incorrect about this sentence: The boy running past the library too quickly.
user
I’m not sure I know what you mean by “incorrect” but it’s missing a period.
assistant
What is incorrect about this sentence: The boy running past the library too quickly.