One of the more famous and certainly curious decisions at the Second Council of the Lateran in 1139 was a ban on using missile troops against Christians. Specifically, Canon 29 states that:
We prohibit under anathema that murderous art of crossbowmen and archers, which is hateful to God, to be employed against Christians and Catholics from now on.
This is frequently misrepresented as a crossbow only ban, usually accompanied by far fetched claims such as likening the crossbow to "weapons of a mass destruction" (more likely, it was an attempt by the Church to take the moral high road). In any case, it seems similar bans were issued by Pope Urban II in 1097 and repeated again towards the end of the century by Pope Innocent III, but I cannot locate definitive sources or texts for either.
Obviously, the bans didn't last. Archers and crossbowmen continued to feature in European battles everywhere. For example, the famed Genoese crossbowmen dueled English Longbowmen at the Battle of Crécy. I even found references claiming Pope Gregory IX employed "Provençal mounted crossbowmen" against the Lombard League in 1239 (but that seems a bit dubious since AFAIK they were allied against the Holy Roman Emperor that year).
What I am curious about, is whether the ban had any actual effect on European warfare (or diplomacy!) at all. Did any feudal lords ever take the ban seriously, or had it been unenforced right from the start? Was there any outrage when the ban was violated, or was it always ignored by the secular world?
Did any contemporaries actually take note of the ruling?