Can I use an LED bulb in a incandescent only fixture?
There are a few reasons you might not want to use an LED bulb in a fixture:


If the fixture is dimmable and the LED bulb is not, the bulb may flicker or not come on.
If the fixture is enclosed, an LED bulb may overheat. (LED bulbs generate much less heat than incandescents, but they do generate some... and they're a lot more sensitive to overheating). Some LED bulbs say they're OK in enclosed fixtures.
LED bulbs are often larger (even if the mount is the same), so an LED might not fit.
If the fixture or circuit runs a small amount of current through the bulb at all times, this can cause a LED bulb to stay lit. This is usually an issue with light switches that have some electronics in them and don't have their own neutral (e.g. a timer or night light). I can't really tell what's going on with your fixture, but this might be the reason.