I have a pretty big one. Short story, not testing the salinity of the water. Long story:
Went to a new LFS that just opened up down the road from my house. Otherwise it's about 15minutes to the next nearest. Well, I used to get RO water, for top off, from the old LFS. So went to the new place, and starting talking to the owner, etc, etc. He asks what tank I have, 'saltwater' as he sells both types. Hand him the jugs and he goes and fills them up in the back. So basically for 6 weeks, I would go and he'd say "more water?" I'd reply "yep." So anyways, the tank gets settled and buy some corals, a nice favia and a plate coral. Add them to the tank and watch them melt in front of my eyes. I had a couple damsels and a LMB that were fine, so I had no idea what the problem was. Took a sample of the water to get tested. Ran every test after the basic battery, came back good. Scratching our heads, he happened to ask what salinity was. I replied, "not sure, haven't tested in a while." So he grabs the hydrometer, and adds my water.
I've never seen an hydrometer arm shoot up so fast, went as high as it could go, the guy wacked it a couple times and it didn't budge. So my salinity was high, something over 1.035+. The guy, turns out, thought I was buying saltwater to do water changes, when I just wanted fresh for top off. After some going back and forth, I walked out with some store credit for the lost corals.