...who don't have the balls to call you or email you and say, "Hey, I can't work on that project that I promised I would do for you because of [insert reason]. I hope you will understand, and will be able to still move forward. I hope I didn't cause too much of a delay on your project."
If you are over the age of 21, and you cannot do this simple thing, then go to the adult store, buy some big boy/girl pants and some suspenders, and put them on and act like an adult. Or, if you are just a big baby, go buy some adult diapers and wear them, if you refuse to grow up and be responsible.
Hell, if you can't return a simple text message within a reasonable time frame (within 2-4 hours, excusing valid reasons for delay), then you are a little too much of a weenie and need to join the adult world.
I know several people who do this, and it absolutely drives me crazy. It says to me (rightly or wrongly), "Hey, you aren't important enough to me to get back to you in a timely manner. So suck it." (Kidding on that last part. Mostly.)
I might get flack for this, but to me, it says the same thing for people who are chronically late. Again, excusing those who have a valid reason: kids, pulled over by the cops, etc. If you are chronically late to every function that I invite you to, it says to me, "My time is more valuable than your time, and it doesn't matter if I say I'm sorry, because I will keep taking advantage of the fact that you don't say a word when I'm always late."
Or, if I extend an invitation to you for various functions, and you consistently refuse - yes, it hurts my feelings. Or you say you will show up and you don't. It's a slap in the face, that after time goes by, the slap hurts less but it will still sting.
What does this come down to? Manners. Yes, it bothers me that so many people don't think anything of being chronically late, or saying you will show up and you don't, or not returning phone/text/emails in a timely manner. It's like, "Oh well, big deal." Well, for some reason, my mother instilled manners in me (more like drilled them into my thick skull) and it bothers the shit outta me when people act like they were raised in a barn (which would sorely disappoint their mothers, I know).