I'm still new to the all-grain scene and I've messed up my sparge more than once, leaving me with a pre-boil gravity that is too low. I got to thinking, if I fall short (in pre-boil gravity) in the future, I could simply do another sparge. Assuming I have room in my kettle, I don't see any harm in collecting a 3rd or 4th running --even if it gets my kettle volume higher than I want it to be. Naturally, I'll have to boil down the kettle until I reach my desired volume, but at least I'll have the sugar I failed to collect from the previous sparge (the water will boil away, but the sugar will stay behind).
As long as I don't throw extra ingredients into the kettle until the appropriate time, is there any harm with collecting additional runoffs and boiling the water long enough to reach my desired kettle volume?