SFist commenter hillarys_quivering_lip tells us about a warning that went out to passengers on a the 12-Folsom Muni line today. Behold:

"Listen up, folks: look before you sit down...someone's been leaving sewing needles in the seats on this line. Three people got hurt yesterday and I don't want anyone getting hurt today..." - 12 Folsom driver Muni Bus #8010 at 8:33 am

Heads down before you sit down, folks. There might me something prickly on your seat. And, of course, if you find anything sharp on your seat, report it to your driver.