Well, I think it deppends on what do you belive.

If you belive that there is only 1 definition of good and bad, then good will never be bad.
If you belive that something good is something useful (meaning, that brings most benefit to most of the people involved), then good will be bad if it's proven that that thing is not useful anymore.
If you belive that something good is useful to yourself, it'll be bad when it's not useful to you anymore.
If you belive that knowing what is good or bad comes from a long experience (spiritual or ethical), maybe a larger experience might prove something to be bad.

Btw, there is calculation on the idea of "useful", in other words, you'll balance the benefits against the isadvantages of it.

yep, It depends on what you believe in. We are born like a piece of while paper, there is no good and no bad. So you have to distinguise black from white, you have to know what's good and what's bad, with you own judge.
Well, telling a tie is bad except for a good reason, right?

Never. Good ends never justify bad means.

What about medical experimentation? I'm actually a strong supporter of animal rights, but I expect that many here would concede that if experiments on a mosquito for example could help find a cure for a major disease, then it's a "good". Or a mouse, although I'd be very uncomfortable with that dilemma and don't know what the right thing would be to do - cure cancer by torturing a mouse or mosquito? The same would be unambiguously wrong in ordinary circumstances, but maybe a utilitarian could argue that it's justified. I'm sure some would insist that vivisection for medical research is not a "bad" in the first place. My point is that the same action may be regarded as good or bad depending on motives and/or outcomes; e.g. sadism or altruism.

... I'm wondering about the dangers, limits, pros and cons of such thinking. Perhaps some recommended readings on utilitarian ethics?

when she's half your age and still over 25!

Hello Deluno.
All our decisions in life are made for a reason, and all of them are censored by our ego. . . even though at times we cannot see how. Usually a bad decision is one not in our favour, also not in anybody else's favour too, especially if our bad decision points back at us (either openly or secretly). So when is bad good? If it is bad for our ego (fabricated-self) then it will always be good for our being (true-self).
The ego sees any truth as bad (a threat) because it knows it's own truth is just a fabrication and not valid. The truth nullifies, humbles the ego (even for a fraction of a second). For many reading this, if you got to this point without suspicion you are doing well. When we lose sight of this truth the ego becomes busy - always seeking further validation - to build up self-esteem. The ego sees anything that could point to its truth as bad. However, it is very versatile in defending itself. It can be anything that will work to its favour, from quick and abrupt to slow and subtle. It is very evasive when it comes to its own truth. The ego comes from our own mind and uses it to try and keep us in denial about it. Much like trying to win a chess game with our self.
So, why is it good for us when it is bad for our ego? Because our truth, as much as we try not to see it at times, is what keeps us on the straight and narrow.
But truth has a price, it has no room for untruths - including our ego. There is an old saying, 'the truth hurts', what hurts is our ego squirming (embarrassment, guilt, shame, remorse, etc).
There are many measuring sticks to use for ethical decision making. One of them is to ask our self a question, 'what is in it for ME?' If there is something, then it is feeding our ego (self-esteem). Oh, by the way, a truth-ful person knows their own truth and has no need for self-esteem.
Remember, our true self needs nothing from others, or from our ego mind.

Bad is never good, by definition. Bad means to a greater good is an oxymoron; bad ≠ good.
Extrapolate; would life be better if everyone was doing something bad for the greater good? I think not.
Yes, at times (very few) it seems, the ends do outweigh the means; especially if one feels no other choice is available. (Where the lack of foresight is always a rouge factor.) If the value one puts upon the end, is so much more acceptable for most all of society, then the less than honorable of means may be used. The problem is assumed values, which are a bit volatile.

If a sailor, who has been at sea for many months, acts sexually aggressive in a bar - he may be given more sympathy for his actions, than a lounge lizard having the same exact behavior. As SiulaGrande pointed out, if one is seen as more bad than the other, you have conditional values. Conditional values can't be used to judge "bad" behavior because it's situational, not fixed. Without fixed values, you can justify anything in the end.
When there is a difference of acceptance of bad behavior, "bad" can beome the whim of opinion, and not a value at all. Acceptance of bad behavior is not the same as denying it is bad As "ends outweigh the means" argument goes - the bad and the good are weighed as one, and the result is judged either bad/good. The level of acceptance given to the bad, weighed opposite the desirable level of the good, is done on a scale, and not found as an absolute yes/no.

When is bad - good? Never, except maybe, when people wants it to be.

To cause pain to a child is bad.
To stick a needle into a child causes it pain.
To inoculate a child against disease is good.

Just ask yourself three things:
1. What am I doing?
2. Why am I doing this?
3. What are the circumstances surrounding this action which might affect either #1 or 2.

Then, of course, you will want to judge whether #1 and 2 are good or bad. Obviously, you'll need some kind of standard by which to make this judgement.

Bad is not good or good is not bad. They are two separate, distinct qualties from my point of view. Good is what the individual values; it is desirable and offers them satisfaction. Bad is the complete opposite, however we are aware that a good deed can result in an undesirable outcome and visa versa.
Hence as time passes there are fluctions of good and bad in the resulting string of events resulting from intial actions considered to possess one or the other ethical quality.
Imagine these qualities may ploted on a graph against time. They are analogous to the seasons. After one quality there is the appearance of the other. Just like winter only appears for a particular period so does good and bad. Its the nature of things, but thats a debate for another season pending your response.

I think it depends on how you define bad.

Relationship with a girlfriend? 8(