I have a bunch of micros. I have a large program, but not so large that I get the "unstable" warning during compile. But after uploading this program, my Windows 10 lap top with 1.6.13 IDE will no longer open the USB port so that I can upload anything, including the boot loader. I had, in the past, uploaded my application after the compiler gave the "unstable" warning, and I couldn't upload again until I reburned the bootloader. Now I can't reburn the bootloader (can't open the port) on any of the micros that I have uploaded my application to. I am using the "arduino as isp" with a mega2560 and also another Micro as isp without success. I get the "Yikes!" message. When the compiler said my program was too big and I had this problem, I was able to upload the bootloader to 5 of my micros using the mega2560 as isp.