It took a social media post for the Philippine central bank to realize its mistake of circulating a bunch of defective, "faceless" banknotes.

The 100-Philippine peso notes ($2) were missing the portrait of former president Manuel Roxas. Two words were also omitted from the country's official name, according to a report by the AFP.

The central bank, Bangko Sentral ng Pilipinas, said in a statement on Thursday that a "mechanical cause" was behind the printing mishaps.

A screenshot of the Facebook post that revealed the defective notes.

"Millions of banknotes are printed by the BSP on a daily basis. In the manufacturing and production process, there are cases when machine errors cause rare misprints. In this particular case, the BSP has identified the mechanical cause of the said erroneous printing. It has since been resolved," it said.

The mistakes were uncovered after a woman's Facebook post showing the defective notes went viral.

The central bank has asked those holding the erroneous notes to return them, but AFP, citing the country's central bank managing director, reported that only 33 such bills have been discovered.

Local news site the Philippine Star reported that the "faceless" bills accounted for only 0.00009 percent of the total banknotes in circulation.

AFP said that is not the first time the Southeast Asian country has seen such errors on its local currency. In 2005, a batch of 100-peso bills with then-president Gloria Arroyo's name misspelled was accidentally put in circulation.

In 2010, currency was issued showing a rare native bird but in the wrong colours and with an incorrect map.

Read the full AFP story here.