Mixing grain and grape
I know of a few instances off the top of my head where American whiskey producers have used grape-based flavoring or wood. First was the duty-free Wild Turkey Sherry Signature. WT finished some 10 year old bourbon in sherry casks and then added a bit of sherry into the final product before bottling.
Second was Woodford Reserve's Sonoma-Cutrer chardonnay finish. WR finished some bourbon in chardonnay casks from the Sonoma-Cutrer vineyard in California. Both companies share an owner, I believe.
Buffalo Trace has produced limited releases of experimental bottles of whiskies finished in zinfandel and chardonnay casks.
North of the border Kittling Ridge's John Hall matures some of his whiskies in casks that previously held sherry he made.
I'm sure there are some more out there, especially from the microdistillers, I've missed.