Occasionally, a word of
masculine gender can be used as a nickname, slang term, or an ad hoc means
of referring to a female. In such instances, one usually follows grammatical
gender, although reference by natural gender may also be encountered. Thus,
˚uczek Bug used as a girl's slang nickname will most often take masculine
agreement.
The gender of
nouns ending in a functionally soft consonant must be determined individually.
For example, mysz mouse is feminine, while kosz basket is masculine.
Such nouns may occasionally cause indecision for Poles themselves.
from that site that was cited few treads ago
I tried to recall other examples, and there are ones ending with c (noc-night) cz (klacz- female horse) ń ( jesień - autumn) źń (przyjaźń- friendship) ć (nić-thread) dź (szadź- hard rime)
As is said above , proceed with caution.