AVR USB Programmer gives error MOSI line shorted to VCC.
Most likely cause is DebugWire is enabled (DWEN fuse set), probably during development process by tools like AVR Dragon or JTAGAVR mkII. You will need to clear DWEN fuse with your DebugWire interface from AVRStudio before ISP will work. For example, with AVR Dragon 1. Debug Menu -> Start Debugging (May now have to wait for code to load into the target) 2. Debug Menu -> AVR Dragon Options -> Communication Tab -> Disable DebugWIRE button