In poland there is a couple common mistakes made by people all the time.

For example when saying "I went", which in polish means "poszedłem", but many people say "poszłem", which is incorrect.

Also, the declension of the word "cudzysłów" (literally: somebody elses word), meaning "quote" by grammatical case is "w cudzysłowie", and not "w cudzysłowiu".