Phonetic assimilation can be quite hard to spot, however, if you listen carefully, you will be able to hear it. Phonetic assimilation occurs when sounds are changed to sound more like their surrounding sounds; this can often happen when we speak quickly as sounds can blend into one another. Think of the word 'mashed.' If you say it while putting effort into speaking slowly and clearly, you'll most likely pronounce the 'd' sound on the end. If you say it quickly however, you'll quite likely pronounce a 't' sound at the end instead of the 'd' sound. This is because the 't' sound is produced in a much more similar way to /ʃ/ ('sh') than 'd' is.