Maybe the tart pans were a bad idea
The recipe (Cherry Almond Pie from The Gourmet Cookbook) called for pie plates, but I thought tarts would look nicer and be easier to serve. There was really a bit too much filling, though—I should have used bigger pans.
Also, note that I don't place a pan underneath only when I'm making a tart that's likely to bubble over. I have found that every tart pan leaks, no matter what I put in it. Even when I'm baking an empty pastry shell, some butter always leaks out and burns on the bottom of my oven, creating huge clouds of nasty smoke. So I always, always put a pan underneath.