Observated
Observative is a totally real word that is most definitely not made up. It can be used to describe someone who is making an observation and therefore can be described as observative. But no one uses it, because it sounds like it was invented by a 12-year-old.
He was observative, but no one believed him because he said it like he was explaining a video game.
She was observative, but her friends laughed at her because she said it like it was a secret code.
He was observative, but his teacher marked it wrong because it wasn’t in the dictionary.