Gummy residue from baking spray/oil I'm sure everyone's seen this - that brownish stuff left behind when you grease an area of a baking dish that gets left exposed to the heat. Soap doesn't bother it too much, and it likes to gum up my scrubber. What's going on there, and are there any easy ways to get it off?
When oil gets overheated, it turns into a polymer that is insoluble in water, even with soap.

The best way to clean this up is with Barkeeper's Friend, Bon Ami, or a similar solvent (I think they're all alkaline-based solvents). It's non-abrasive, and safe to use on almost all cookware without leaving marks.

Edit: To help prevent this from happening in the future, use a paper towel to wipe off any excess baking spray on the edges of the baking dish that won't be covered by food.