In Matthew 5:22, Jesus said, 'whoever is angry with his brother without a cause shall be in danger of the judgement'. A small number of Greek manuscripts omit the words 'without a cause' (P64, 01, B, plus two other late manuscripts). P64 is the oldest manuscript at this place, but these three manuscripts are all Alexandrian - that is, they are textually related to each other, so it is not surprising that they have the same reading. The vast majority of manuscripts contain the words. When we look at the versional evidence, not only do the Old Latin and the Syriac versions contain the words, but even the Coptic versions contain them too, breaking ranks with the three Alexandrians, their stablemates.
In terms of transcriptional probability, the omission is only one word in Greek, and the fact that scribes tended to omit single words rather than add them means that the omission is far more likely to be the result of scribal oversight, especially when there is a case of homoeoarcton involved, like here, where the following word starts with the same letter as the word in dispute.
In terms of theology, the Bible makes a clear distinction on the subject of anger: it is sometimes right and proper for us to be angry, while anger can also sometimes be sinful. Thus, Jesus was angry (Mark 3:5), God is angry with the wicked every day (Psalm 7:11), and the Christian is told to 'be angry and sin not' (Eph. 4:26). The words 'without cause' in Matthew 5:22 seem to nicely capture the right balance and the retention thus seems more in keeping with what the Bible elsewhere teaches.
Thus, although antiquity favours the omission, on the other hand majority, propinquity, theology and transcriptional probability strongly favour the retention of the words. It is hard to see how the words, if they were originally missing, could have spread to so many different geographical regions in exactly the same form if they had been added in by inventive scribes taking liberties with the text. It is more likely that one extremely limited stream of related evidence accidentally omitted the word.