The latest issue of the Journal of Biblical Literature (129:3) has a great article entitled "A Disconcerting Prayer: On the Originality of Luke 23:34a" by Nathan Eubank. "Father forgive them, for they know not what they do" has been considered a spurious reading by modern textual critics, because the textual witness is divided and the prayer is just "too good" to have been omitted as a hard reading. Yet Eubank makes a thorough case that this prayer was indeed a hard reading in the early church for a number of reasons. I'm reasonably convinced by his arguments.
In the course of this study, Eubank highlights a problem with the text-critical axiom "the harder reading is to be preferred":
Hort warned that in making use of transcriptional probability, “we have to do with readings only as they are likely to have appeared to transcribers, not as they appear to us.” Yet Hort’s own confidence that no scribe would have omitted something as sublime as Luke 23:34a illustrates the tenuousness of arguments based on one’s general knowledge of early Christianity, rather than on actual early Christian interpretations of the reading in question. 
So if someone wants to make an argument based on the concept of a harder reading, one had better know what was considered hard when the text was being transmitted. This seems like a daunting task at first, because in most cases there is no direct evidence as to what any particular scribe or patron considered hard (unless there is comment in the margin). Furthermore, given the variance of opinion which has always existed in the church, there is never a safe assumption about what any particular scribe would have considered "hard," even if we can determine a general consensus in the church on a particular matter. So, how can a "hard" reading be determined?
As Eubank demonstrated, this can be done with references to the church fathers, among other methods. And the particular power of Eubanks' argument comes in that this same textual change is witnessed in all text families and time periods, a pattern which can be effectively explained by a "hard reading" being omitted by some, and not others, outside of the typical patterns of textual variance.
So while I'm usually skeptical of arguments from difficulty, Eubank has highlighted a very good example, and at the same time vindicated the textual validity of one of Jesus' most interesting prayers.
 Eubank, Nathan. "A Disconcerting Prayer: On the Originality of Luke 23:34a." Journal of Biblical Literature 129:3, p. 536.