The job did not start as a particularly interesting one. One minor noble craving status wants a particular painting; another noble doesn't want to sell said painting. The former asks around in seedy taverns after the infamous Owl of Sternhill, offers to pay a modest sum to arrange for the transfer of property. There are guards, but not terribly bright ones. There are traps, but not terribly dangerous ones. There's a celestial lion - that's novel, at least, but still not much of an inconvenience. And then there's the private art gallery, every piece bedecked with alarm spells. Not even very good alarm spells, none of them trigger when dispelled.
So, there they were, bag of holding filled to the brim with jewelry and idols (no reason to take only the contracted item, and when security is this bad why not?) eyeing the item they came in for - a painting of some overgrown forested ruins that no reasonable person would pay even a third of Malak's commission for, contemplating whether to try to fit the whole frame into the bag, or take it out and roll it up. Taking it out would involve either putting the frame back up, empty, or leaving it on the floor, empty, which would be sloppy and kind of unaesthetic. Buut the bag is kind of full, and the frame was not all that nice.
They give it a last look-over for traps, find none. They take the frame down, open up the back. They put on gloves, because snooty nobles don't like their expensive artwork being touched, and grab the canvas to start rolling it up.
It is about half a second and one awful sucking sensation later that they realize they really should have done another check for magical traps after they'd opened up the frame. Now they're stuck, by the looks of their surroundings, inside a painting that was actually possibly worth a little more than they thought.