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.

2008 Chemaxx Hypochlorite plus Urea Explosion.pdf