I lost my streak even though I did my lesson.
I was on day 99, about to be 100. I did my lesson and was excited to know tomorrow I would get to 100. I logged on today and found that it said I didn't do my lesson and my streak was lost! I didn't buy a streak freeze. I am very upset and I don't want to pay $9.99 for something I should already have.
The three most common reasons for this are:
you're working on a phone or a tablet, and your phone / tablet hasn't been able to synchronise its data with the server.
your daily target is set to more XP than you did yesterday.
you've travelled across timezones, and Duolingo's using the time zone you travelled from.
The first is the most likely - that your mobile device hasn't synchronised.
It's always a good idea to buy a streak freeze if you care about your streak, and to check every now and then that it's still active. It's also a good idea to use the website to check that your phone / tablet has synchronised with the server.
Actually, my target goal is set to 1XP. I try to do at least 50XP a day though. I'm going with the first one. It really annoys me though. I actually thought I already had a streak freeze, so now I'm wondering how many times this happens.
EDIT: Also kinda annoyed because I was about to have a 100 day streak.