Nothing new can cause something existing and publicly known or published or patented before the filing to infringe.
It can happen that A + B, interconnected in a certain way, achieves something that was previously thought to need C. The new invention is doing it without C. Or maybe it has a result or function not related at all to the result of the known A+B+C device.
Assuming the examiner finds the reference to A+B+C, A+B could only be allowed if the claim either limited itself to the case of a new and non-obvious way to interrelate A+B.
That is called a negative limitation and would need to be well supported in the specification-as-filed. If the inventor didn't know about A+B+C they probably don't explain how they got around needing C and would find it hard to add "without C" at the last minute.
If A+B was patented and I invent an improvement A+B+C, I will need permission from the A+B patentee to make. sell, import, etc. A+B+C whether or not I get a patent on that combination.
Also note that products infringe claims, patents do not infringe other claims.