Dear LaTeX,

I wonder whether this is a bug that should go to the LaTeX Bug Database, 
since it only
-- is about an internal
-- contradicts what one might expect
-- while it is not clear to me whether this can affect LaTeX's function on 
the user-level

What do you expect from

     \in@{ionization}{ionizat}\typeout{\ifin@ YES\else NO\fi}
     \in@{bonbon}{bon}\typeout{\ifin@ YES\else NO\fi}
     \in@{client-to-client}{client-to-}\typeout{\ifin@ YES\else NO\fi}

The problem is generally characterized on pp. 9f. of

Perhaps this is known (concerning \in@) and has been taken into account 
when it was used.

I also don't know how to produce a LaTeX mistake from this (using user or 
package writer commands only). A hint on this may be that the problem 
already occurs with patterns that end on the same character as they begin 
with, e.g., \in@{msam}{msa} (this idea because \in@ seems to be mainly used 
in handling fonts, while there seems to be no danger with searching 
comma-separated lists).

LaTeX bug or not?

Happy TeXing,