Windows 10, Windowed Mode, JRE 1.8.0_25
Using a flat-world for Redstone testing/projects. Blocks and items involved are; Observer x1, Slime Piston x1, Furnace x14(surrounding moving slime and Redstone block), Slime Block x3, Redstone Block x1, Redstone Dust Line x4(2 under lever, 2 on upper furnaces), Lever x1. While testing an Observer based momentary switch system with the 3 slime and 1 Redstone block stacked respectively near piston vertically, I had stopped the circuit using the lever and noticed 2 extra slime and 1 extra Redstone block next to my original. I've restarted minecraft and was still able to replicate this bug. Obviously some sort of duplication bug, which could be used in a survival setting as well. The first screen capture is the original setup(http://prntscr.com/fh65n4), the next one(http://prntscr.com/fh65cj) is the duplicated blocks.(NOTE: I did try attaching these screen captures but I kept recieving an error no matter what browser I used and no matter what the extension was(jpg vs png etc) ///JIRA could not attach the file as there was a missing token. Please try attaching the file again.///)
Edit: I was able to produce faster results using the same machine from both sides of the slime and redstone blocks. It seems to duplicate when the piston is moving too fast and the redstone powers at the same time as the movement. It almost appears like the blocks are being pulled out of the others like a rip in space-time-tic(haha).
MC-54026 Blocks attached to slime blocks can create ghost blocks