Or one engine crit and a fried side torso. If your armor levels aren't that high, it's not as uncommon as you might think.
A CXL engine saves ~15% of the Mech's mass, and needs 4 critical slots. CF-F only saves you 3% of the Mech's tonnage, and uses up 7 critical slots. If I had access to the factory, I'd take a Mech with CF-F, and swap it for CXLE. This saves me 11-12% of the Mech's tonnage, and frees up 3 critical slots. Even with a 20-tonner, that gives me 2 more tons to play with for extra stuff. Including extra armor.
CF-F is a great Field refit for a Mech, so the Mech's protection can be increased much faster (or saving a ton or so and adding more ammo/weapons), but the CXLE is a wonderful thing, that has to be refitted at a Factory.