Here's a partial list I had fun thinking up: lamb, plumb, dumb, tomb ~ Before you read the rest of the post, list any others you can think of without consulting a dictionary.
Now here's a thing. These pairs work in English: crumb/crumble, plumb/plumber, limb/limber, climb/clambor
These pairs do not work in English: limb/limble, lamb/lamble
And while we're at it, I wonder why the heck the first of each following pair isn't actually a word:
mumb/mumble, nimb/nimble, ramb/ramble, tremb/tremble, grumb/grumble, tumb/tumble, stumb/stumble, rumb/rumble, thimb/thimble, humb/humble, shamb/shambles, fumb/fumble, gamb/gamble/gambol, bramb/bramble, jumb/jumble, bumb/bumble, bimb/bimbo