Over the years I have found or purchased muzzle loader guns that were loaded. One was a shotgun I inherited from my grandfather. It had been stored in his attic for close to 60 years. Upon cleaning it up I ran a check of the bore, and sure enough it was loaded. Got the wads and shot out, but had a difficult time removing the hard caked powder. Finally a soaking in a bucket of water for a day soften it up enough to flush out.
The other gun was one I purchased through Gunbrokers. It was a 58 cal Investarms Hawkin. I checked the bore as soon as I unwrapped it from the packaging asn realized right away that there might be a load in the barrel. Using a ball puller, I extracted 2, conical slugs and the powder I estimated at 100 grains. I would not have wanted to be on the but end of that gun if the load was fired off. It pays to check all muzzle loaders, especially older ones, cause it was common practice to keep a loaded gun in the corner or some other place convenient.