I often wonder why some objects can't be placed on top of another while others just don't mind at all? Grids are one of these that really shouldn't be placed on top of one another but don't seem to annoy Revit at all when they are. Then we are left to figure out why a Grid number isn't available but we thought it was. Then it turns out that someone accidentally added a grid on top of another one and we can't see the grid value of the one hidden beneath it. A little head scratching and we are back on track.
Still, wouldn't it be more effective if Revit at least said, "Umm Steve, I hate to distract you but you just put a grid on top of another grid. Would you mind getting a clue and a life and fix it for me?, Cheers mate!". Better yet if it said it out loud in the voice of Mrs. Emma Peel (Diana Rigg). I can take criticism from her surely!