As for dealing with other people, it's subjective. If they're not satisfied with your answer, it's an excuse to them.

To be a bit more specific, I'd say there are two factors at play, which are of course hard for the other person to judge, especially if they're a manager not involved in the task itself:

• If you can reasonably perform the task as expected despite the obstacle, it's probably an excuse. If you can't, it's almost certainly a valid reason.
• If you wanted to perform the task as expected, then something that caused you to not do so is likely a valid reason. If you didn't want to, then you're more likely to be using an excuse.

Of course, it also depends on the priority level of the task. If your sibling asks for a glass of water and you get them a mug because there are no glasses in the cabinet, those stakes are low enough that it's a valid reason even though you could have checked the dishwasher or washed a glass yourself.