When subjects sign up for studies that are scheduled at a convenient time or have an exciting title, this can?

I believe the answer is: Bias the sample

Convenient timing and biased title would most likely make the study to appeal to a specific segment of population, which could make a the sample become biased.

For example, researchers could make the majority of their subjects to be younger in age if they conveniently timed the study in the middle of the night, when most middle aged and older people are already asleep.