The folks over at LensRentals stacked together a whopping 50 UV filters, and slapped them on top of a Canon 5D mk II with a 400mm f/4 lens. The resulting images look like you're staring at a scene through a bucket of swamp water.
What's also interesting is they compare the clarity of shooting through a stack of five high-end UV filters against five inexpensive ones, and the sharpness difference is dramatic — though less noticeable if you're only using one.
They're not the only ones to have tried stacking their filters to the sky. Spotted on reddit this is an immense stack of lens filters, sixteen or so by my count, strapped to an old Konica. User ImitationBacon claims he was the one who put this together, saying "I was very bored one night, and I found a bunch of my grandfather's camera equipment, and took a picture for the hell of it. I really didn't think somebody would be that inclined to save it."
Another great example comes from a user who stacked together 10 filters on his Pentax K10D — and they weren't just UVs. He added a Vivitar Color Correction 81B, Hoya HMC 82A, Toshiba B131 (80A), Minolta UV, Hoya Half-Color, Sears ND (Neutral Density) 4, Tiffen 4-Point 2mm Star, Tiffen Fog #1, Tayon CU (Close-Up) +2 and a Promaster Spectrum 7 Polarizer. He then took a photograph of a tree, which resulted in a blue and red monstrosity he proudly called "the most horrible picture in the world".
I'm just waiting for the super-blurry and unintelligible looks of these filter stacks to be included in the next lens pack for every Lomo mimicking application in existence.