I have found 2 possible bugs and/or mistakes.
I have encountered one drill a few times where the recording tells the time, but instead of saying HACHI, she says HICHI. If I encounter it again, I will look at the source and tell you which drill number it is.
The other one is sometimes when doing drills on mobile, I answer a question, but instead of going to the next drill, it takes me to the home page.
Anyone else encounter this?