Bleaching, curled up, excess sliming among others. Best way to avoid that is put less light demanding corals lower in the tank shaded if possible and "light acclimate" the whole tank.
Allowing the full intensity of the light to gradually come up over a period of weeks is best when switching over to MH
. Most prefer to use screen door material layered several times. Every few day you can remove one layer which slightly increases the intensity. This also allows you to maintain a normal photoperiod and less disruption to the normal routine