Pronunciation of "c'est"
Something that often throws me off is I'll hear "ce" instead of "c'est" in the machine dictions. I had always thought "c'est" sounded more or less like the English "say," but it seems like Duolingo sometimes goes for a short rather than long 'e' sound.
Is this consistent with real spoken French? Maybe I'm just not hearing the difference yet.
Edit: And while I'm thinking of it, another one that trips me up constantly is de/des. I expect "des" to sound like "day" but often hear "de" instead, and end up making an error.
c'est is more like get (more open). des is more like "git" (more closed) but can be a bit more open, depending on the mood of the machine. The pronunciation is not always consistent.
Someone asked a different question about the difference between the pronunciation of "blond" and "blanc." I recommended trying this voice synthesizer: http://www.acapela-group.com/text-to-speech-interactive-demo.html
It has four French voices and it is really helpful for distinguishing pairs of sounds.
Here is a link to the other discussion: http://duolingo.com/#/comment/26262. I hope you find it as helpful as I and the other student did.
@Mizotte acapela is remarkable. You can get an acapela widget with your choice of voice and then put it on a web page: http://www.acapela-group.com/text-to-speech-widget-blog.html
I hope Duolingo knows about this product. Their product's price varies by how it is used.