An al-Qaeda affiliated rebel group in Syria is reported to have asked for forgiveness after beheading a fellow rebel in a case of mistaken identity.
A video recently posted online showed members of the Islamic State of Iraq and the Levant (ISIS) brandishing the severed, bearded head of a man.
They said was an Iraqi Shia caught fighting on the government side.
But other rebel fighters watching the video recognised the man and said he was one of their commanders.
In a separate development, activists said a senior figure in another Islamist rebel group had been killed in an air strike near Aleppo.
Youssef al-Abbas of Liwa al-Tawhid was reportedly meeting the brigade's leader, Abdul Qader Saleh, and another senior figure, Abdul al-Aziz Salameh, at a rebel-held air base on Thursday when he was killed, according to the opposition Aleppo News Network.
It said Youssef al-Abbas, also known as Abu al-Tayyeb, was Liwa al-Tawhid's intelligence chief, but the UK-based Syrian Observatory for Human Rights described him as the group's financial officer.
Mr Saleh and Mr Salameh were wounded in the attack. ANN reported that they were taken to a hospital in Turkey and were in a good condition.
Video footage posted online by activists purportedly showed Abu al-Tayyeb's body being transported to his hometown of Mareh.
The video of the beheading shows two ISIS fighters in Aleppo province - one holding a knife - brandish a severed, bearded head, denouncing their victim as an Iraqi volunteer for President Bashar al-Assad's forces.
They decry his immorality, saying he is a heathen - one of those who have threatened rape of men as much as women.
BBC Arab affairs analyst Sebastian Usher says horrifying videos like this stream out of Syria every day from the rebel and government side - each with the aim of sowing terror.
What makes this one stand out, he adds, is that other rebels watching the video recognised the beheaded man.
Members of the hardline Islamist rebel group, Harakat Ahrar al-Sham, said he was not a government fighter at all, but one of their own. They said he was a commander called Mohammed Fares Maroush.
He is believed to have been wounded in the recent battle for Base 80, south of Aleppo, with government forces and was being taken for medical treatment by rebel fighters.
But our correspondent says he seems to have thought he had been captured by government militiamen from the much feared shabiha. The militia is dominated by Alawites, members of the president's heterodox Shia sect.
Mr Fares is reported to have offered prayers that would have made him seem a Shia rather than a Sunni - and therefore on their side.
This was heard by ISIS fighters, and in the growing atmosphere of sectarian hatred, he was then subjected to his savage fate, our correspondent adds.
The Daily Telegraph quoted ISIS spokesman Omar al-Qahtani as conceding the error.
According to the newspaper, he alluded to a saying by the Prophet Muhammad that Allah forgives those who kill a believer by mistake.