/kʊdnt/ → /kʊbm/, /wʊdnt/ → /wʊbm/, etc.
The assimilated new phoneme can produce a domino effect which triggers further processes of assimilation. This gives rise to very interesting cases, not at all infrequent.
Have a look at this example:
Now follow the different stages of the process:
- First, the /t/ dissapears, which can happen because of elision -the article about elision is not written yet, but coming soon- or just because the /t/ is frequently dropped in negative contractions.
- Then the /n/ becomes a /m/because, now we no longer have a /t/, it is followed by a bilabial /b/.
- Finally, the /d/ is turned into a /b/ because now the next sound is a bilabial /m/.
- So, /kʊdnt/ becomes /kʊbm/.
And here there are some examples from real life:
My take on this is that, had the potato not failed again in the summer of 1846, we wouldn’t be here talking about the famine at all (Cormac Ó Gráda, BBC4; notice that he’s an Irish speaker of English).