Timer underflow glitch (infinite time!!!) & No incorrect answer glitch
I don't know if it's really worth pointing out because they don't really interfere with duolingo. They're more like available cheats, but since it's a self learning course I don't see the point of 'cheating' to begin with.
Anyways, I found that, if the timer has two or less seconds left and was paused to load the audio (slow internet only?), it is possible to cause the timer to underflow by typing something random before the audio file actually loads and submitting it.
When the underflow occurs, instead of taking me to the 'time is up!' page, it sends me to the next question with timer set to 23826855:59 or something around that number every time. It only seems to happen at a very specific situations- I tried to replicate it a dozen times, and succeeded only half of the trials.
Also, I noticed that, by disconnecting from the internet after starting a practice, every answer will be marked as correct except for multiple choice questions. If the internet is connection is recovered before answering the last question, it will successfully load to the score page with all the marks earned without giving proper answers.
(EDIT: the image file wouldn't attach for some reason, so I'll just put a link: http://postimg.org/image/z7c17ghal/ )