There are tests (t/db_dependent/Koha/CurbsidePickups.t) that generates the "Wrong slot selected" message:
#Day is not ok 
Cannot create a pickup on a day without opening slots defined
#Day ok but datetime not ok
Cannot create a pickup on a time without opening slots defined
# Day ok, datetime inside the opening slot, but wrong (15:15 for instance)
Cannot create a pickup on a time that is not matching the start of an interval