You don't say which cylinder this plug comes from but if it is the most awkward one to get to on your car I'd say that the plug spanner/socket you're using tends to slip at the point that the plug comes loose. Easy to do and I've done it myself. Sockets with a rubber plug retainer need to be pushed down hard or they only just mate up with the flats on the plug and that can make it slip off when you put some pressure on the lever that you're using. This will sometimes break the ceramic or bend the top as it has here. Some plugs are easy and you can get a straight pull on them but if you've got a servo and/or an AC pump there, access isn't so easy. So, maybe try a different tool, a short box spanner and tommy bar perhaps, even the one supplied in the tool kit if you still have it? Plugs only need to be tightened to about 25ft/lbs with fresh washers, which is only hand tight plus a bit extra.