I had a late walker who was constantly tumbling (and hitting his face, natch) when he first started. Once he got a bit more stable on his feet it seriously decreased, though I'm still a bit paranoid. Aside from free play (which is usually downstairs in my injury resistant zone), I keep him in his chair unless I'm right there next to him - dressing to go outside, when I'm prepping snack or craft, etc.

If it makes you feel better, it's happened to me too! And I get the guilt, for sure. Just keep doing what you're doing (as I'm sure you're a great provider and have a safe environment), and try to not to dwell on it. Debrief and have some wine tonight!