"Always report nonreproducible errors; they may be ticking time bombs. Nonreproducible errors can be the most expensive bugs your company might release. Sometimes the program misbehaves in a way that you can't replicate. You see the failure once, but don't know how to get it again. If that happens to a customer, it will erode confidence in the product. Programmers have tools that you don't have. If you report the symptoms clearly, the programmer can often trace through the code, asking how you could have gotten a certain message or what could possibly have happened when you were looking at a given dialog or clicking on a given control. It is not unreasonable to believe that programmers can fix *20 percent of the 'nonreproducible' bugs that are reported to them."
*Obviously, the percentage can vary across products and toolsets available to the programmers.
Excerpt from the book "Lessons Learned In Software Testing".