Worst debugging issues are always things I can't access directly, on top of being rare.
Think network appliance in the middle that don't log or not at the level you need (and sometimes they can't log what you need).
Those usually mean that no reproduction is possible, except in production or very close to it, with tools you don't always control.
Annoying ones are those of "This http request is sometimes slow", and chasing each boxes in the middle shows a new box that is supposed to be transparent but isn't, or some rare timing issues due to boxes interacting in a funny way.
Think network appliance in the middle that don't log or not at the level you need (and sometimes they can't log what you need).
Those usually mean that no reproduction is possible, except in production or very close to it, with tools you don't always control.
Annoying ones are those of "This http request is sometimes slow", and chasing each boxes in the middle shows a new box that is supposed to be transparent but isn't, or some rare timing issues due to boxes interacting in a funny way.