I put a guy off running Spacemaster ever again by doing exactly that at the start of the campaign on a self-replicating alien killing machine whose machinations were going to be the focus of the entire campaign. It turns out that AT20 300DB and -3 to all crit levels wasn't enough to keep it alive for more than a round against a lucky roll...
[also this was my first successful genocide committed during an RPG...]
Also: Crits and fumbles are great fun, but it's equally fun and much less hard work to just read through them all, rather than play just for the sake of funny crits.
You can always tell RM/SM players by the way they snigger in ANY percentile system when someone rolls a 66...