It is if the Hadeeth has an omission from the end of the chain; then if it is after the the Taabi’ee is Mursal, and this action Irsaal, like the Taabi’ee saying “The messenger of Allah, prayers and peace be upon him said…”
Mursal ahadith are da'eef in principle, since we do not know the person from whom the tabi'ee related from is a sahabi. Although there is some ikhtilaaf on this issue, as there is an opinion that they are saheeh on the condition that the tabi'ee is thiqqa, and also a third opinion which is somewhere in between the two - that it is accepted after certain other conditions are met.
Sorry if I don't make sense, and I messed it all up.:X