Murdering People in the Masaajid! The Khawaarij committed this heinous act in some of the lands
Shaikh Rabee Bin Haadi (may Allaah preserve him) said: The Khawaarij kill the people, whilst they are in Salaah. They (carefully determine) the time of the Salaah and kill people in their Masaajid. This has taken place in some of the lands- they (i.e. khawaarij) launch an attack against them and kill them, whilst they (i.e. the people) are in Salaah.
[Source: الذَّرِيعة إلى بيان مقاصد كتاب الشريعة (See 1st paragraph) 1/135]