I remember as an apprentice, doing a three month stint in the powerhouse; 4 off turbocharged Mirrlees engines similar in size to the pic I have found (attached).
One engine had a complete top end overhaul; all heads removed. It was off line for some time, and every component was laid out on tables and accounted for.
When the last head was bolted back on, there was one nut, and washer left over on the assembly tables (couldn't be accounted for).
Every head was removed again as we investigated.
Every nut and bolt had been re-fitted during the assembly, someone had mistakenly placed the extra parts on the table.
No place for carelessness in critical work environments like this.
ps; before you ask, it wasn't me!!!
Richard