I agree in both cases it is rude not to respond. But in one case the potential employer is the one who reached out first. For example, when I got out of college I brushed off the fact that an employer didn't respond to my resume at all. That didn't make me angry. What did make me confused was someone who specifically reached out to me to try to hire me through email and a phone call and then fell off the face of the earth when I followed up via email with a resume and more details. Sure that person might have been busy and had more important hires to make than a post-grad, but it was still rude and I probably won't forget it.

Yup. I had a similar experience in college when I applied for an internship at Facebook, got back an enthusiastic-sounding response asking me for my availability for a phone screen, and then the recruiter dropped off the face of the earth and never responded to me again.

I agreed to a job offer from a company, and by the Friday before I was to start, I hadn't heard from them on where exactly I was supposed to show up on Monday morning. I emailed them about it, and they replied, "Oh, oops, we neglected to inform you that we restructured our existing staff and don't need you any more."