You can 'break in' to levels you haven't yet reached (bug report)
When you are in a certain skill (e.g. dates and time), even if you've only just started the skill, you can 'break in' to the last level of that lesson by appending the level's number to the url, as in http://www.duolingo.com/skill/es/Dates-and-Time/7. Upon completing that level you receive a skill completion message even though you haven't completed the entire lesson. The next skill doesn't get unlocked though.