flub vs stumble

flub and stumble both are nouns.

Word	Noun	Adjective	Verb	Adverb
flub	Yes	No	Yes	No
stumble	Yes	No	Yes	No

As nouns, stumble is a hyponym of flub; that is, stumble is a word with a more specific, narrower meaning than flub:

flub (noun)	stumble (noun)
an embarrassing mistake	an unsteady uneven gait
	an unintentional but embarrassing blunder

flub (verb)	stumble (verb)
make a mess of, destroy or ruin	make an error
	miss a step and fall or nearly fall
	walk unsteadily, tripping repeatedly
	encounter by chance