Which is a lot easier to remember for a test than when working on your deathless prose. . .
Sometimes I even manage to forget to put in what it looks like (to the POV's eyes, of course; I stick very closely to, sometimes on the verge of stream of consciousness). What it sounds like is more difficult to remember, especially when dealing with local color and not with sounds significant to the plot.
Touch and smell -- well, touch comes in on temperature reports. But smell is something else, hard to remember, hard to imagine, but very probable. Taste you can forgive yourself for overlooking; it's very rare.
Plus of course there is the effect of coming back to something you wrote at least a week ago and discovering what you wrote instead of what you thought you wrote. No matter how vividly you imagined it, it can come across rather flat.
Then, that's what revision is for.