"he's a smart kid, he just needs to apply himself" and "but you know what to do here, I've seen you do it so why are you having problems with it now?"
Both are things I've heard constantly through my life. Made me feel horrible about myself every single time because I didn't understand why at the time either.
Most of my teachers just assumed I was pissing about behind their backs and would ask me "you must know why you haven't been able to complete the work/homework/task" even if I had been listening and really trying.
Took the whole of primary school and the first year of secondary school before I actually got a teacher who understood and helped me to actually get somewhere.
Still not officially diagnosed just because of how shockingly bad adult ADHD diagnosis is in the UK and not being able to go private but most all the techniques I've used have worked and I think almost any GP that would look at me would probably agree.
Just to say, anyone who might be reading this and can imagine themselves saying/have said these in the past, please don't. You have no idea how unhelpful/damaging talking like this can actually be to someone who really needs the extra support.