I was surfing around ad found this photo, and immediately wondered how something like that might happen
After thinking about it for a minute, the answer is simple. Final Protective Fires. Final Protective Fires are used when a military unit is in a static position and is in danger of being overrun. If you're in a good defensive position with open fields of fire, it's easy to interlock fires where all the units weapons are shooting in interlocking lanes of fire.
Eventually, if you fire enough, over the course of several years of combat, you're bound to have two bullets slam into each other.