At the end of each sprint, it is important to keep the following in mind:
"When determining velocity count only finished stories, that is, stories that pass their acceptance tests. Do not count stories the team partially completed during the iteration."
It is too easy to just pretend that things are going better than they are, so the important thing is to remain realistic and just tell the truth.
Otherwise you are bound to accept a technical debt which will only add up and confuse things before you realize it. No use struggling at the last minute when it can be avoided far in advance.
Be careful, be brave, and report correct information, despite the deadlines.