I found such a situation in my neighborhood today at the Table Mesa Mall. You can see that I have added parking lot, parking aisles (not very straight but I will clean them up), buildings, shops and even stairs.
My scenario is that I am leaving King Soopers grocer on the west side of the complex and want to travel to Savers on the east side. I put the query to both open.mapquest.com (which uses OSM data) and maps.google.com (which uses data from somewhere else). Open.Mapquest sends me via the shortest route through the parking lot... Google sends me around the periphery.
Clearly, as Google gets more data into their dataset, their routing will likely get better... but that's the point, isn't it... that OSM also didn't have the data but then I came along and added it... and then MapQuest took that vector data and updated their routing web service.
Go OSM!!! Good job MapQuest!!!
Here are the two results (you can click on the image to get a larger view)...