A  municipal water treatment company decided to switch from chlorine gas (Cl2) disinfectant to liquid 12.5% sodium hypochlorite (NaOCl).
At one of their remote facilities, they had a 2500-gallon polyethylene tank that previously held a liquid fertilizer consisting of 78% urea and sulfuric acid (1:1 ratio). Someone in management decided that they could reuse the liquid fertilizer tank for the sodium hypochlorite, most likely to save a few dollars on a new tank. 
During filling, the tank exploded, and the site supervisor and other employees present were seriously injured. The site supervisor died within about 48 hours.