Never assume! Always check for yourself! Even if it’s with a pen tester.
I had a very similar thing happen to me when I was an apprentice. It was a new construction at our smelter, we isolated and I was about to jab my screwdriver onto the 347/600v lug, and my journeyman, who doesn’t usually do things by the book, stopped me and said ‘hang on a sec’, and for no particular reason, he just wanted to show me good habits I guess (which is very unlike him). We took the time to measure and lo and behold the switch was live. The contractors who built it put the wrong labels on the buckets in the switch room compared to the labels that were in the field. I was literally an inch away from a serious shock. I was shitting bricks for a few weeks after that. That taught me to always check for myself, no matter what. Hopefully you learned that too! Stay safe out there.