When you are doing the Rescue for Fort Neugrad, and are attempting to sneak into the fort via the underwater entrance on a character in full heavy armor and zilch in Sneak.
You're doing pretty well, and are just about at the lake, when your sneak indicator turns to full eye open and stays there even when you stop moving. There at the entrance to the lake, with no way around, is a person. I'm like, did a Stormcloak somehow get over there? But then I slink closer, expecting "Failed: Sneak into the fort / Free the prisoners / Take over the fort" to show up at the top of my screen any moment now, and it's an Imperial captain, who can clearly see me, but seems not to mind.
"Clearly this is just an off-duty soldier out for a midnight swim in the lake, being sneaky just for the Oblivion of it. Stormcloaks? What Stormcloaks? There is no possible way this could be some stealthy Stormcloak trap. Inconceivable. By all means, sir, please proceed to the lake."
Possible explanation: I have Voice of the Sky active right now; earlier there were hired thugs outside of Whiterun who refused to attack me until I actually talked to them.