Occasionally, using both the default bard song, bard song 16, and bard song 15 (I can grab their names if needed), my bard will do 1d4 (divine, positive, piercing) damage vs giants, as a temporary effect on his rapier. It doesn't happen every time, but I'd guess every third or fourth?
Still happening. Checked with another bard using the same songs, no such weirdness.