Every 30 or 40 repetitions, a word will come up twice within a few minutes, the system apparently having forgotten the first time.
For instance, 5 minutes ago 逼供 came up. I got the tone and characters correct and moved on. After a few more repetitions, it came up again, the timestamp inaccurately reading: "studied 28 hours ago".
This has probably happened 10-20 times this week, and I haven't seen it before. It's not a big problem, but a little disconcerting that some of my work might get lost. FWIW I'm using Chrome.