PetroStreet

Normally, all pressure vessel and heat exchangers which have to be operated on high pressure service have butt end nozzles and are hydro-statically tested in shops by welding temporary caps on nozzles. After successful completion of hydrostatic test, manufacturer ships the same without removing nozzle caps as they provide protection to butt end nozzles' face during transportation.
Now when they reach at site, in order to attach piping to the nozzle, cap is required to be removed. Vendor provides you with the drawing of cap attachment on nozzle which should be ideally followed in order to be sure of all the dimensions before starting the cutting activity. Collar of 5 mm is normally provided by manufacturer just in case the grinding cut goes deep but should not be affecting nozzle body & reducing pressure design thickness.
Drawings not read properly and taking this activity as a lesser critical one can result in nozzle damage as shown in the attachments.

These damages require very sensitive repair activities and then stringent NDTs depending upon the code being followed. Re-hydrostatic test after repair and NDT is normally advise for ASME coded vessels.

Nice sharing.
I would like to go a little further.
Sometimes, vendors also provide some welding built-up on the nozzle face in order to facilitate field welding at site, avoiding post weld heat treatment but that's also got missed if you do not refer the drawings.
I have witnessed such observations where the welding has been done assuming the nozzle material same as the attachment piping resulting in repairs and damage to the buttering thickness.

IN the specific case how were the repairs done? What NDTs were employeed for ensuring the integrity of vessel?
Was the vessel stress relieved?