Fighters from an al Qaida-linked Syrian rebel group beheaded an allied commander whom they mistook for a pro-government fighter, activists say.
It was the latest excess attributed to the aggressive and radical Islamic State of Iraq and the Levant.
While allegations of ISIL attacks on civilians and infighting with more moderate rebels are commonplace, this time it appears to have killed a member of another hard-line group, Ahrar al-Sham.
A militant website reported that the ISIL acknowledged the incident and asked for calm, saying that mistakes occur in wartime.
The incident happened in the northern city of Aleppo, where two fighters were videotaped displaying the severed head to a crowd on Wednesday. The men claimed the head belonged to an Iraqi Shiite fighter allied to the government of President Bashar Assad, but residents later recognised it as belonging to a rebel commander.
Activists said on Saturday the ISIL fighters found the wounded rebel in a hospital after a battle with government forces. As he emerged from anesthesia, he called out the names of saints recognised by Shiites, an Islamic sect whose members have largely sided with Assad, one of the activists said.
Assad is an Alawite, a member of a Shiite offshoot sect, and Shiite militiamen from Lebanon and Iraq have fought alongside his forces against largely Sunni rebels.
The incident underscores the sectarian nature of Syria's war and the chaos among rebel groups battling Assad's forces in their quest to overthrow the government.
"The medics knew he wasn't a Shiite, but they were too scared to stop them," said one of the activists.
The next day, two young ISIL rebels were videotaped brandishing a man's head.
"This is an Iraqi Shiite who fights for the army of Bashar (Assad)," one man shouted, holding the man's head in one hand, and a knife in the other.
But residents recognised the head as belonging to Ahrar al-Sham leader Mohammed Marrouche, known by his nom de guerre Abu Abdullah al-Halaby, one activist said.
The Aleppo-based activists asked to remain anonymous for fear of retribution by ISIL.
Furious Ahrar al-Sham partisans posted a video calling ISIL fighters "idiots" and noted that rebel fighter Marrouche had longish hair and a full bushy beard typical of conservative Sunni Muslims. "The Shiites do not do this," they sarcastically noted. Others posted a video showing Marrouche calling for his group and ISIL to work together.
On Saturday, a pro-rebel website published a poster attributed to Ahrar al-Sham that showed a close up of the two ISIL rebels, with the word "wanted" emblazoned across their faces. "Two killers wanted for judgment in the Islamic court," read the poster.