Incorrect Query Result
Aside from invisible characters, the possibility of an OS patch changing the results of such an query exists through a change in locales.

An index involving text types is only valid if the underlying collation never changes. Postgres relies on the OS for comparing strings, through the libc, so a change in an OS collation may have the effect of an index becoming unusable, as if it was corrupted.

If there is an index on the column, you can test this hypothesis by issuing SET enable_indexscan TO off; before the offending query and see if it gives the same results. This check may also reveal an index corruption independantly of any problem with the collation.