Oh wow, I did that recently! Over 40 years of building models and I finally did it. When I cut with a straight edge and X-acto I always stop and check my finger positions. I finally forgot to do that and cut the tip of my left index finger clean off with a fresh out the box Excel #11 Super Sharp blade. Oh dear, that smarts!

Emergency room, sew it back on, didn't take, follow ups. Throw in the strange shape it has healed into, the lack of any sensation other than extreme pain when you bump it and yep, cutting off a finger tip costs way more than 25 cents. Even if you throw blades away as frequently as I do, a new one almost every use.

Dave