Which are common mistake in English language?

Well, not to be rude but there is a mistake in your question, instead of just mistake, it would be mistakes because it is plural. The most common mistake I think in the English language is sheeps and fishes. The plural is sheep and fish. But when you are referring to multiple kinds of fish the word fishes can be used. Another one is octopuses and cactuses. This would be cacti and octapi. Although, octapouses is getting more and more comon and now is somewhat grammatically correct.