You know what, I knew it was wrong when I was typing it but I just couldn't remember what the right way was so I just left it. So thank you for this.

Wait. So is this or is this not an eggcorn?

https://en.wikipedia.org/wiki/Eggcorn

It was indeed an eggcorn, and I knew it was but it had so completely replaced the correct phrase in my mind that I just looked at it like 'that's not right, but I don't know what exactly is wrong about it'