Hello. I'm Hirashin from Kyoto, Japan.
Yup. Some Japanese people write funny sentences either intentionally or unintentionally.
As you may already know, most people avoid using the structure as in 面白いでした. But some people might say that for fun though that would not sound right.
Also, I know some young people who say すきくない or きれいくない, which should be すきじゃない/すきではない or きれいじゃない/きれいではない.
If you are not a skilled speaker/writer of Japanese yet, it would be better to follow what your grammar book says.
Later,
ひらしん
