have I simply been lucky so far?
No, you have damaged the arduino pins and reduced the life of both the arduino and the LED. The fact that so far it is all still working is beside the point, one day it will fail and you will have forgotten why. The up side is that it is probably only those pins that you have overstressed and the rest of the arduino will be undamaged. Also you will not have damaged those pins when you use them as inputs.
Do a search on this forum and see how many people report that there system suddenly stopped working for "no reason", that is probably a result of past abuse that the thought they had "got away with".