I might just be slow, but I like to keep it clean and control my altitude in one place (that would be the MCP). What would happen if I am slowly climbing in V/S to 5000 ft enroute to a fix with a 4000 ft hard altitude. I forget to follow up pressing ALT INTV once, twice, three times.... Then cleared up high to FL170 and press VNAV? It will probably show an error message and disconnect VNAV? Just speculating through the grapevine.