One of the favorite arguments that creationists love to spout is that genetic mutations are always detrimental, and that there is no such thing as a beneficial mutation. They often challenge skeptics to give them examples of even one beneficial mutation.
Of course when they are given examples, goalposts are immediately moved to discard them.
For example, if you give them the example of the mutation that protects some people from malaria, they will refuse it as a "beneficial mutation" because it has the side-effect of causing propensity for sickle-cell anemia. Of course "beneficial mutation" must mean that it has to be only beneficial, without drawbacks. It doesn't matter if there is some benefit, it doesn't count. (This is actually one of the examples that creationists love to attack and laugh at. Never mind that sickle-cell anemia isn't nearly as bad as malaria.)
But fine, it's not like we have a shortage of beneficial mutations. We can give them examples of mutations that are purely beneficial, without drawbacks. For instance, you can give them the example of Stephen Crohn, a man who had a quite unique genetic mutation that rendered him basically immune to HIV. The standard response of creationists to this is denial, doubt ("I don't believe it") or dismissal using some really lousy excuse, such as that it doesn't count because the mutation wasn't passed to further generations, or something.
If the uniqueness of the mutation is such a problem, then one can give them an example of a very widespread mutation: Lactose tolerance. It's quite well established that humans were lactose intolerant in the distant past (and many of them still are), and a relatively recent mutation made a significant portion of the human population tolerant. There are many benefits from being able to consume milk in adulthood, and basically no drawbacks.
Again, the standard creationist response is denial or doubt (ie. they doubt that it's actually caused by a "mutation"), or clinging to some perceived drawback as an excuse to dismiss the example. And if anything else fails, they will move goalposts once again and fall back to another favorite tactic of them: They will say that this mutation didn't "add new information" to the gene pool.
If they do that, then you have already won the argument. The original claim was that there is no demonstrable beneficial mutation, and we have given them an example they cannot refute, except by falling back to a completely different claim. (Of course, good luck trying to make them admit that their initial claim was false. Don't hold your breath.)
The demand for purely beneficial mutations also shows another misunderstanding that creationists have about evolution. They seem to think that evolution is possible only if there are purely beneficial mutations that have no drawbacks whatsoever. That's not how it works.
Species change all the time. Some changes are good, some changes are bad, some changes are neutral. Moreover, most individual changes to the genes actually produce several morphological changes. Genes are such contrived and complicated constructs that changing even one single unit of it will often have several different consequences (many of which might not even be apparently related to each other at all.)
Many beneficial changes will, as a kind of side-effect, also produce less-beneficial changes. However, if the overall result is that the species becomes more fit for survival, this change will more probably remain. In other words, if the benefit outweighs the drawback even a little (in terms of chances of survival and procreation), then it has a good chance of being retained in the gene pool.
Thus, it doesn't matter if a mutation that eg. protects from malaria also causes anemia: If being protected from malaria increases survival rates compared to the anemia, then it's overall beneficial and it will more likely be retained. After all, evolution is a completely mindless process; it has no goals (like "aiming for perfection"), it just happens. Sometimes species get horrible defects alongside the change that makes them fitter for survival, but evolution doesn't care. It can't care. It's just a side-effect of natural phenomena.