Glad you weren't hurt! I had a "new" grinding wheel years ago, and since then, whenever changing wheels on ANYTHING, 4", 6", 8", bench grinders, angle grinders, cut-off saws, get out of the line of the wheel and run them free for ONE FULL MINUTE, and this does not guarantee anything, but it helps. People also have a tendancy to pick up tools, drop them, not say a word and put them back as found, I have came back from restroom, etc., picked up an angle grinder, ( that I was using a minute ago ) switched it on and had the wheel break immediatly.
SAFTEY IS HARD WORK, BUT WORTH IT.
Keith