Old 3.5 gallon toilets and why they clog.
One reason are bowls like this.
Notice that it's not designed to siphon really well.
And then notice the shelf on the floor side, and where they cut the outlet. You would be surprised to know how many times these are pulled an something long has slide right past the hole and lodged beyond it.
Thanks Terry. That cutaway makes other improvements in modern designs for squeezing more power out of less water very evident. Back then the siphon jet was plumbed off the front of the rim, making that long trip, and losing pressure all the way. Many modern siphon jet designs are fed directly from the input to the bowl, with one or two tubes running along the outside for higher pressure at the jet. Notice also the sharp right angle turns at the beginning of the trapway and the end of the siphon jet tube, rather than the smooth sweeps used now.