Occasionally, /s/ and /j/ assimilate into /ʃ/ not across word boundaries but within a single word. Listen to these two interesting recordings:

 Is it possible for us to look back and assume that Gatsby’s world was normal? What do you think about the setting that Fitzgerald gives him (Melvyn Bragg, BBC4)? /əˈsjuːm/ →  /əˈʃuːm/

 And there was an ensuing debate in The Letters Page lasting for several weeks about modern poetry (John Goodby, BBC4). /ɪnˈsjuːɪŋ/ →  /ɪnˈʃuːɪŋ/

 

If you want to learn more about assimilation, click here:

Introduction to assimilation