At one time information was released suggesting that a small amount of gasoline mixed with diesel fuel for use in cold weather was an acceptable practice. This information was eventually retracted due to conditions that may result from such a mixture.
First and foremost is a safety factor. The gasoline fumes from gasoline are very rich and there is not enough oxygen available for a spark generated at the filler neck to ignite these fumes. The diesel fumes are too lean and will not ignite. A mixture in the right proportion could create fumes volatile enough to ignite from a spark generated at the filler neck.
Second, all injection pumps and injectors utilize the lubrication in diesel fuel to keep internal parts lubricated. Gasoline cuts down on this lubrication and can lead to premature injector and pump wear or even failure.
Although accidentally mixing a small percentage of gasoline with a tank of diesel fuel may not result in an immediate problem, it could still result in shorter pump and/or injector life. The only safe action would be to drain the mixture and start from scratch. While this may seem costly, it could save you from more costly repairs in the long run