What common grammatical mistakes people make in English?

many... but here are some ;) 

• The common phrase "how are you?" is often answered by "I am good." However, good is not an adverb and cannot describe a verb, including am. Good is an adjective and describes a noun. A more grammatically correct response is "I am well."

• The difference between these two words lies in their different grammatical functions. Lie is an intransitive verb, which means that it is never followed by a direct object. If a direct object is required, lay is used. A person cannot lay down, but he can lay the book on the table before he lies down.

• Like lay and lie, these two words are often confused because they communicate a similar idea, but are used in different grammatical contexts. Who is a pronoun that takes the place of a subject; whom takes the place of an object. Whom is the appropriate replacement for a direct or indirect object or the object of a preposition. Therefore, it is never "who did he ask?" or "whom did it?" but "whom did he ask?" and "who did it?"

The most common mistake i always hear from my work mates daily 

when i ask someone " where is (name) ? "   and he replies : " He didn't came " :)