It makes sense that carb ice can happen at high humidity – lots of moisture means lots of icing potential. But, according to the NTSB, it’s possible to pick up carb ice with relative humidity as low as 35% at glide power. Not a problem you want when turning base to final.
The X-31 flight test team was the “A” team — the best people, from every discipline — from every organization. But they lost an airplane. If it can happen to the best team, it can happen to any team.