What can actually happen to a turbocharger? Give it some thought, not only is it a part which rotates very quickly, it is also a part which has very tight tolerances. Thus, neither the turbine- nor the compressor wheel can handle any sort of foreign objects. Apart from that, whereas other mechanical components, e.g., in the engine, may break down quite slowly, with the turbocharger it all happens in a matter of milliseconds.
So, this means that leaving screws that have not been tightened to the correct torque or leaving washers in the intake port, will possibly be the death sentence for the charger. Should the charger have been repaired or replaced, the whereabouts of further objects is to be scrupulously ascertained. This of course, is also valid for shavings and carbon residue and concerns, e.g., the intercooler, which in such cases, should ideally, be replaced at the same time.
Indeed, most of the time, self-made errors concerning the intake- or output port, don't happen very often. There is more often the possibility of foreign bodies being sucked in because of faulty air-filtering or through a defect in the intake manifold. Other causes could also be: a blocked catalytic converter or particle filter, which would lead to the charger overheating, the engine may have been over revved or chip-tuning may also be the reason.
Presumably, you've given little thought to how a turbocharger can be over revved despite the engine being run at normal RPM. Through a leak in the feed to the engine, the motor-management e.g., could request more charging pressure and thus, cause the charger to over rev. Once again, this a case where under no circumstances, only the charger should be replaced, the reason and any further effects caused by the fault must be found.
Thus e.g., the tale of woe concerning the replaced charger, may also be continued because tolerances have not been adhered to. Not only did that produce shavings, it had also supplied the intake with a large amount of oil. This oil may, under certain circumstances, not even have got as far as the engine, where it could be burned off, but has got trapped in the intercooler.
Speaking of oil: It could of course be, e.g., that because of a blocked oil-line, too little oil is getting through to the turbo-charger. This of course, as mentioned before, will very quickly lead to overheating, which can be seen by the blue colouring of the shaft. This is of course, accompanied by scoring and further destruction.
One important point is, basically, we're talking about the turbocharger and not the engine itself. Their oil circuits, although connected to each other, must be considered separately. If the oil-line to the charger is partially- or completely blocked, it concerns only the charger and not the engine itself. The engine itself is in danger, when parts of the charger start flying around!
So, while oil which is dirty or too old can be a danger to the supply, the free flow of the oil can also be hindered. Where then does the oil go? Into the oil-sump, of course. If here, the pressure is too high, through the compression passing the piston, the oil flows more slowly, perhaps too slowly. Indeed, also the faulty installation of sealings or the excessive use of sealing paste can narrow down the return flow. 09/14