Yep, it’s the problem of not knowing what you don’t know.
You see a GT’d sentence and notice a mistake in it. “Wow, how embarrassing!” you say, feeling proud of yourself for knowing enough Japanese to picked up on what was wrong.
Meanwhile there’s another mistake that completely changes the meaning of the sentence, but since you don’t know about it, you just assume it’s fine.
One time I was trying to convince someone that GT was harmful to his learning process, but he wouldn’t be convinced: he believed that it was a reliable tool for verifying his own translation. Eventually I gave up and told him 好きにしな (“do as you like”). Well, it turns out that both this guy and GT thought this sentence meant “I don’t like it”. He came to a (wrong) conclusion about the meaning, validated with GT, and found out that they agreed – so he assumed he was right.
You know you can paste whole sentences into Jisho and it’ll break them down into individual words? It’s not perfect but it’s a hell of a lot better than Google Translate.