The B sits there quietly at the end of the word, doing nothing.
subtleSUT-ul
The B disappears entirely.
debtDET
Silent B, from the Latin "debitum".
doubtDOWT
Silent B, same Latin root as "debt".
combKOHM
Silent B after the M.
thumbTHUM
Silent B after the M.
climbKLYM
Silent B after the M.
lambLAM
Silent B after the M.
crumbKRUM
Silent B after the M.
plumberPLUM-er
Silent B, even though "plumbing" keeps the sound in some related forms.
limbLIM
Silent B after the M.