Black spot on subject faces

[Douglas]

I have had several instances of a dark spot as if the person had a black eye show up on both my Canon 40D and 60D when using flash, indoors. Could that be caused by the sunshield being left on the lens? 

[ScottyP]

Do you mean "lens hood"? 

An improperly narrow lens hood  would cause a dark ring around the edges of the photo.  If the hood has "leaves" or "petals" it could have dim spots in irregular places. 

Is this a Canon lens?  If so, are these the correct Canon lens hoods for the particular lens?  If you have a mis-matched lens and lens hood, that might be an issue?

But another issue could be unflattering shadows from on-camera direct flash?  One's oen nose or forehead will cast ugly shadows on a subject's face.  You probably already know this, but if you bounce your flash off the ceiling or a neutral wall, then it decreases these shadows a lot. 

Also helpful is the use of a reflector.  You can pay $60.00 for a collapsible reflector, or you can get the same one on Ebay (Chinese) for like $10.00)  Position the reflector to cast bounced flashor reflected sunlight  up onto the subject and you are good to go. 

[7D5D]

In addition to wrong or improperly installed lens hoods as ScottyP mentioned, your lens may be blocking some of the light from your built-in flash. For example, if your lens is fully zoomed out and/or your subject is fairly close, you might be casting shadows on the lower part of your subject. Also keep in mind the coverage of your built-in flash, both cameras cover up to 17mm so if you are shooting wider than that, you will have some light fall-off.

Could you post some samples of the black spots?