plurked, then switched to private plurks and clicked the "1 new plurk [update]" and had his own public plurk appear on his private timeline
happens all the time
it's ok as long as it doesn't goes vice versa - private plurks appearing on the public timeline 
it's only a glitch on your timeline, doesn't reflect what others will see apparently
doesn't believe that others will see it, i think it's just client side shenanigins