would have to be the infamous "Every third Computer is fried" issue, where a Commercial Building's Service is derived from a 240/120V 3 Phase 4 Wire Delta system.
One particularly odd head-scratcher was back in 1989, on a new "Ground-Up" project (multi unit Commercial Warehouse complex).
Services were 208Y/120V 3 Phase 4 Wire, ranging from 800 Amps to 1200 Amps.
Gear consisted of UGPS, Main Disconnect section, with Three or Four Meter Sections attached.
Went to shoot trouble per a Tenant's complaint of very dim Fluorescent lights, and some equipment's power supplies losing smoke.
Issue was Open Common Grounded Conductor (aka "Neutral").
After close to an hour of searching, the problem was not poor make-up in J Boxes or at Outlets - it was at the Meter Sections!
forgot to tighten the Bolts for the Bus Jumpers between the Main Section and the first Meter Section.
happened to be the guy I was working for at that time.
Found out later that He was a major Pot Head, who also decided to start using Methamphetamine.
This, along with keeping His Head inside of His own A$$, may have been the reason for not tightening those Bolts!