There are dozens of different mistakes that are made every day in grammar, even by professionals, including mixing up words like who and whom, using its instead of it's or using apostrophes incorrectly. Sometimes these mistakes are unconscious and can be caught when reading back through the writing, but sometimes the mind sees what is supposed to be there and not what is actually there.
Turning in writing for work or publication with these common grammar errors can make work seem unprofessional and create a bad impression. Most of the common grammar errors apply to the written word, but they can be found in the spoken word as well. If someone writes affect when they mean effect, it changes the meaning of what they are writing and can affect the impression that co-workers and other have of the writer.
Many issues arise when text or online jargon mixes in with everyday discussions or writing. Words like "literally" are used in pop culture to exaggerate a situation when trying to describe it, but when writing a report, corresponding in a professional manner or giving a presentation, avoiding these types of words is best. Stating an exaggeration or pop culture reference in a presentation can be amusing but may not make the best impression on those receiving the material.