Glyphs allows parts of the feature file to be disabled; glyphs2ufo ignores this setting and copies them to the features.fea anyway, with the result that:

featurePrefixes = (
{
code = "lookup IDontCare { sub nonsuchglyph by nonsuchglyph; } IDontCare;";
disabled = 1;
name = disabled;
}
);

produces a features.fea:

# Prefix: disabled
lookup IDontCare { sub nonsuchglyph by nonsuchglyph; } IDontCare;

When the feature is disabled because glyphs have been removed, this can cause the font to fail to build. (Also it's just wrong to include feature code that the designer has asked to be excluded.)