This is something special. Bugs are distilled out of findings (see Q-Course by Bob Legrand), and can be classified in different categories that would be worth it's own article. First, we always find a priority for this Bug. This is important to prioritize all bugs in the never ending bug report. Define the amount time the bug has to be fixed in - yesterday is the demand, but difficult sometimes. Then, find out the time you really need (and don’t forget personnel, material that are hopefully available in time referring to ITIL again). The principle to do this is simply a short version of COCOMO or an Excel sheet with some estimations. Lucky, is the project manager who has enough review material to use for this estimation. But all that is fishing in the clouds. Unfortunately there is one way best known as the "Captain Kirk Way": "Scotty, we have a problem", "Yes, I am still working on it", "Scotty, how long will it take to repair it", "Captain, it will take at least 3 weeks", "Scotty, I need it in three days", "Yes Captain, I will try to get it in 1 day, but I cannot promise it". And what happens on the Enterprise? It's fixed in less than 5 minutes. Up to Scotty's last sentence we know the game, only the 5 minutes - solutions are unknown.
The worst thing you can suffer from is a Heisenberg Bug. Werner Karl Heisenberg, a German Physicist, is well known for his Uncertainty Principle that says, that you know either the position or the energy of a quantum particle. I must confess that his contributions to the Molecular exchange force, S - Matrix or Quantum Field Theory are much more fascinating. However, he wrote down the Uncertainty Principle that is relevant for our Heisenberg Bugs. This category of bugs behaves like quantum particles. You know where it was or what it did, but never both in the same time. Even if you know both, you don’t know if it will happen again at the same position in the same form. Chasing for Heisenberg Bugs is an art. You must decide wisely how to do this! It’s not easy because these bugs are not able to be reproduced, they can be of no matter on the customers machine and they can be the end of your project.
Reproducible findings - and therefore reproducible bugs can be found and corrected in a fairly predictable way. Fix them first, always. Sometimes the Heisenberg Bugs disappear when other bugs, or whatever kind of findings you deal with, are fixed. The better the reproducibility of a finding, the earlier it should be fixed. Such requirements are simply implemented. But never underestimate the Bugs with bad or strange reproduction behavior.