[Polkageist]

I'm definitely in the camp of improvising critical failures, since they're pretty rare and often times the table doesn't quite hit the mark on making that rare occurrence interesting in the right way. That said, simply making it an auto-miss keeps things trucking along if there are other things to be paying attention to.

On the general topic of critical failures, and magic specifically, if a table is to be customized for the user then it may be a good/fun exercise to have the player set their own. A fire-mage could work out fire-themed failures, a healer has health-related ones, etc. A bit of tuning afterwards will help make sure it's balanced, but it shares the work with the player and you get a nice bespoke thing out at the end.

As for what a critical failure means though, I had the notion while reading the thread that the severity of the failure should probably scale with the height of the stakes.

For an immediate and non-job situation where life and death are on the line (i.e. combat, adventuring things), then the crit will affect life and death (extra damage, inability to defend).

For a job-like or 'routine' situation where severe injury and/or loss of resources are there (pro atheletes, crab fishing) then things like a broken machinery or chronic injury such as a torn ACL would be appropriate. Costly, and is the 'bad thing' that's not unheard of in those professions.

For the same 'routine' things where the stakes are low, like at an office or a non-hazardous environment setting, then the crit would be on the level of embarrassments or errors that make you have to miss lunch or stay late at work. Lost paperwork, flat tire, missing tool, that sort of thing.

I think with that sort of scheme, a lot of everyday annoyances that you see crop up you can start to map on to the 'that was a crit fail on a job roll' in a sensible way. An airline pilot's crit fail isn't a plane crash, it's a misplaced preflight form and now the flight's delayed an hour. Or, a surgeon's appendix removal crit fail isn't a dead patient, it's a poor suture that causes a bad scar and an unhappy patient.
1-in-216 events being a catastrophe is hard to grasp, but I can totally buy that someone dorks it while making a pizza 1-in-216 times at dominoes.

For magic... well, if it's an everyday spell, then maybe wizards just have a propensity for having odd hair color as part of their daily life. A 'light a candle' spell won't summon a demon, but you're fingernails are green now. Ugh. What a drag.