You should never mix cleaning products. Period. But you should especially never work with an ammonia-based and a bleach-based cleaner at the same time. This is most likely to occur in a bathroom or kitchen, so carefully check labels before getting to work. When vapors from ammonia and bleach mix together, they form a toxic gas that is extremely dangerous. If you discover that you, or someone in your home, has been exposed to these fumes, leave the area immediately and call 911.