There are two potential problems. One is that the more corrosive E85 will corrode parts not designed for it. I doubt many people have any hard information about this, how long metallic and rubber parts on your car will hold up to Exx instead of pump gas. To further complicate matters, you could add something like 2cycle fuel to prevent this corrosion but again, it will be hard to locate information like this. The state if Minnesota did a test with E20 specifically on these concerns you can find by searching with google. This is more a long term issue, of course, one tank won't cause problems.
The second is the fuel trim. Stock, your injectors probably won't pulse enough fuel to run on E85. Many cars can do upwards of 50% stock, but at some point your ECU won't keep adding fuel, and you'll get a check engine light and probably a 420 code. There are piggyback ECU's for most cars that will lengthen the fuel pulse for use on E85 to compensate for this, but they're not free.
Best of luck. A mix of E85 and gas will likely be fine. Not sure why you're doing this, but best of luck and let us know what you find.
