Setting expectations too high can backfire.
You had the best intentions when you told your client she would have the document in her hands within 24 hours. But you ran into a technology issue and then were delayed by a colleague not turning around his part quickly. Now that the 24-hour deadline has passed, your client is dissatisfied with your service.
The irony is that she may have been perfectly happy to receive the report in three or four days if you hadn’t been the one to set her expectations to receive it within a day.
You can prevent this situation by building in a buffer when conveying timelines with clients—especially for activities you don’t totally control. Also be realistic about other demands on your time rather than telling clients best-case scenarios. Better to under-promise and over-deliver than the other way around.