I've hit one in an IM but never anything shorter. There is a direct connection between length of training and when you hit the wall. That's why some Marathon training plans have you go over distance or get close to your projected marathon time. If I'm training for a stand alone Mary...I'll go out to 4:30 or maybe 24 miles...Haven't hit the wall in years.
If an 18 miler is your longest run prepping for a Mary, of course you're going to hit the wall about there...Why didn't you train for the last 8 miles?
Tri's can be the same, although it's different for everyone and of course it depends on the race length...Sprints are easy to train for (to finish) while IM's are in another quadrant of the galaxy. (brief nod to this weekends "Star Trek" opening) Let us know what's on your plate and we can get bit more specific!