I threw an uncursed potion of visibility at an invisible Ogre Magus, and I saw the text that implied it would be visible.
But it was not visible!
I (l)ooked at the spot it should be, and the Ogre Magus was described to be there.
Moved, still didn't show.

Then hit ^r and the refresh showed it.

In big room, blessed Seven League Boots, ratling bard.