Maybe you've dealt with The Escalator before. The conversations with The Escalator go something like this:
"Now, what about this scenario? This is a very high-priority scenario for our users. We have to have this feature in the next release".
"We costed that scenario, and we don't have the time in this release".
"But this is a high priority scenario."
"Every feature in this iteration is high priority".
"You don't understand, our user's can't do their jobs without this feature. If we don't deliver this, the project will fail".
"But, it was never that important before…"
"Look – if we don't deliver this – I … will … lose … my … job".
<blank stares>
"Please! For the love of God! The mafia will kill my family!" <tears>
"Um … ok. We might be able to bump something and squeeze it in…"
"Great! Now, about this other scenario. This is a very high-priority scenario for our users. We have to have this feature in the next release…."