Many common "grammar mistakes" are worth letting slide. Singular "they" fills the gap left by English having no gender-neutral singular pronoun. "Fewer" is technically for things you can count and "less" for things you can't. "That" introduces a clause the sentence needs, while "which" introduces one it could lose. "Further" is for figurative distance and "farther" for physical distance. And "irregardless" is widely used, even though it is considered nonstandard. Language changes with use, and dictionaries follow.
via ‘Further’ Versus ‘Farther’ | Grammar Girl
via irregardless | Urban Dictionary