For ages I've had in my GPT-4o custom system instruction "Be concise, give short answers, BE CONCISE PLEASE, SHORT ANSWERS FOR THE LOVE OF GOD" or similar
And basically it stopped having any effect at all, standard GPT-4o loquaciousness, and I forgot all about it.
Try o1 last week: it's incredibly terse, like really obnoxiously refusing to tell me more than the absolute bare minimum. And I'm like this is rubbish!
Then yesterday I remembered the custom system prompt, removed it, and it's all fixed.. oops
So that's one thing o1 is better at, following custom instructions