One time I was reloading some 44 mag for an upcoming shooting event. I took a break in the action, leaving a tray full of primed cases with the powder dropped into them setting on my bench. I came back a couple of days later and put in the bullets.
At the range, I started shooting the ammo. Bang, Bang, pooft. The first two rounds fired rendered the satisfying and familiar sound and feel, but the third round wasn't right. Rather than pull the trigger again, I open the cylinder and check the barrel. Sure enough, the bullet's just past the forcing cone in the barrel. I break out the cleaning rod, and back the bullet out of the barrel.
I start shooting again, and the same thing happens. Now I get curious. I put that batch of ammo away and continue shooting with other stuff I'd loaded on a different occasion.
Back home, I start pulling bullets, and discover that in some of the cases, the powder has clumped up and their is oil residue in the case walls. After considering this singular circumstance, I decide to investigate further. I notice that my bench has a fine coat of oil over it, except where the reloading tray was sitting.
A few hours later after some interrogation, it was disclosed that the 4 year old daughter was down roaming around the gun room (not a particular concern), but she decided to get the spray can of rem oil out and spray down the bench. A good amount of the oil wound up in the open cases. Wet powder does not detonate.