I always try to constrain sketches to axes but sometimes you can't I have to design some quite complex iparts & further into the development I have to make changes. Whenever I get the response 'constraint to unresolved reference geometry, my heart sinks, returning & amending the affected sketch it seems impossible to pursuade Inventor that there is no problem with the sketch. I can view constraints, remove the geometry causing the problem, constrain the sketch lines etc to new geometry, the lines are all black ie no unconstrained elements as soon as I return to the model the warning returns. the model is fine by the way complete & stable. In the end I usually have to completely delete the sketch elements project the necessary geometry again & reconstruct the sketch. Either that or ignore the red cross, something I find difficult to do!
I think that sketches are the achilles heel of Inventor rather than developing more & more fancy stuff, going back to basics & creating a proactive sketch diagnostic & even say an auto repair or even a method of overriding the error message when the sketch is fine, working OK & producing the model element required. The present sketch diagnostic frankly is a waste of space & time it just does not pinpoint the actual problem & give you the tools to fix it or fix it automatically.

OK Rant over but I'm sure that I'm not the only person frustrated by sketch 'funnies' & the time wasted trying to fix them!