Gun Safety Rule #3 states that:

3. ALWAYS keep the gun unloaded until ready to use.

However, it is not always possible to know for certain if a gun is unloaded or not.
Therefore, one must always assume that the gun is loaded and must always follow Rule #1 which states that:

1. ALWAYS keep the gun pointed in a safe direction.

Refer to Gun Safety LESSONS LEARNED: Rule #1 which includes that:

A "safe direction" ALWAYS includes knowing what is BEHIND any objects that the gun is pointing at AND to consider ALL THE PATHS that the bullet might take after traversing through or ricocheting off of each and every one of those objects.

In this "LESSONS LEARNED", a muzzle loading shotgun was discovered to still be loaded, most likely after over 100 years.
While cleaning the barrel of the Flint Miquelet to Percussion Conversion Shotgun, the author discovered that the gun was still loaded. The photo below shows the wadding and shot removed:

The gunpowder had turned to a hard paste that can be flushed out after soaking in Powder Solvent or WD-40.

The Flint Miquelet to Percussion Conversion Shotgun is shown in the photos below:

Also see:

Gun Safety LESSONS LEARNED: Rule #1
Gun Safety LESSONS LEARNED: Rule #2