I consider myself reasonably fluent in both systems of measurement, and for the normal range of temperatures in north america I can at least ballpark temperature conversions. I'm less good with volumetric measurements but most of my cup and spoon measures have dual measurements, and we have a kitchen scale so generally it's not a problem.
BUT.
Sometimes I'll be converting something and just... forget to stop? Like I'll just keep converting a measurement back and forth, and keep rounding it until it's useless.
Example, the studs between walls in the US are spaced 16 inches apart, so I'll think to myself, hey, two studsworth, that's 32 inches, that's about 3 feet, which is a bit under a meter, so 3 feet, or 36 inches, which is about 90cm, so I need to drill my second hole 90 cm from the first one.
And then I'm shocked when I miss the stud completely? Because I could have just measured and converted? But for some reason my brain gets stuck in this weird loop where I just have to convert everything?