Here is a good explenation by Snarf at EF.

"The problem with mixing the oil and water based AAS is that it forms an oil/water emulsion, like Fonz stated. This is where basically tiny droplet of oil encapsulate water droplets. Normally, if you injected the water based gear alone it is absorbed and dissipates from the injection site very quickly. Therefore, no infection. When it is encapsulated by oil, the water stays at the injection site for a much longer period of time, which can result in an infection. The oil based gear will hold the BA so that it can't start growing anything, the water based gear will not, the alcohol leaves the water and makes for nice place to bacteria to grow."