When the torque converter is fully stabbed into the transmission, the T/C is stopped by the drive lugs of the pump rotor, or inner pump gear for a 400. These drive lugs have bottomed out in the slots that are cut into the end of the torque converter hub. A rookie may partially stab a torque converter where the input and stator splines have engaged, but the pump drive lugs are riding on the end of the torque converter hub and have not dropped into the grooves of the T/C hub. A rookie would install the transmission, tighten the trans to the engine, force the T/C around, not paying any thought to the fact that the T/C is jammed against the flexplate, and will bolt the T/C to the flexplate. The T/C is so well jammed between the engine and transmission, that the flexplate is bending towards the engine. Quite a bit of force to do that. Possibly even the pilot of the T/C is bottomed out against the end of the crankshaft. The rookie puts some fluid in and starts the engine. Finishes filling the trans and goes for a test drive. If he's lucky, he'll make it back to the garage. But, eventually, the transmission will neutralize and not go into gear. The reason this happens is because the spring action of the bent flexplate is pushing the T/C back against the transmission's pump drive lugs that are still riding on the top of the T/C hub. The pump rotor or inner gear are pushed back into the pump cover and the cover wears at a very fast rate until the push force has stopped. Usually, that means the pump rotor or gear has ground into the pump cover anywhere from 1/8 to 1/4". With the excessive face clearance in the pump cavity, the pump becomes inefficient and will no longer build pressure. The funny thing is, when this occurs and the transmission is rebuilt and installed properly, the engine (in my experience) never has needed attention. Bear in mind that the bent flexplate was pushing the T/C back against the pump rotor or gear, AND it was pushing the crankshaft forward. For every action there is an equal and opposite reaction. A basic law of physics.
This explanation is meant to show how much more sensitive the front pump of the transmission is than the crankshaft thrust in a situation where both have been mechanically jammed and a thrust force (equally) is applied to both.
If the crankshaft thrust surface is damaged, but there is no sign of the pump rotor or gear being forced back into the pump cover, then the thrust force that damaged the crankshaft was NOT mechanically born. It was caused by the torque converter being forced forward from the transmission by hydraulic action. Ballooning of torque converters is caused by hydraulic action also, but is a bit more complicated than just the pressure that is pushing the T/C out the front of the trans.