I don't mind discovering a task I've assigned someone is actually way more complicated and time consuming that I expected (although I'd appreciate an explanation so I can recalibrate my assumptions), and I don't even mind finding out a team member I've assigned a task to is not yet skilled or experienced enough to understand the best way of doing it or to complete it in a reasonable timeframe.
What _does_ bug me (as RachelByTheBay alludes to) is the complete radio silence where I've been kinda expecting the job to pop out of the queue completed for the last few days/weeks, only to finally after giving you the benefit of the doubt and not wanting to seem like I'm micromanaging you - discover it's barely been started and doesn't look like it'll _ever_ get completed.
If it's way more complex that I assumed? Tell me. If you're way out of your depth? Tell me. If you think it's a stupid task and you're not going to do it and hope it goes away? Just fucking tell me!