I accidentally used way too much dye (it was very green) and dyed a tributary AND the mainstem of the river fluoro-green for about an hour. I was checking floor drains. Found the one that went to the creek (1/4 mile up from the River), whoops!
Yes, we were trying to trace a water flow that disappeared underground, but I was young and inexperienced and didn’t know how strong the (blood red) tracer dye was. Not only did we manage to make the banks of the Black River look like the aftermath of massive slaughter, our survey ship on Lake Erie (which was using something similar to map current flows as part of our efforts to clean up the lake) detected it the next day maybe 50 miles away.