This has happened to me a couple times. It's quite irritating to "sort of" schedule a lesson, and then sit around hoping they accept 10 minutes before the scheduled time. Or maybe they never accept and then you're stuck there. I don't think iTalki should allow teachers to accept that close to the scheduled time. It should be something like 1 hour before or at least 30 mins before. That way, you can plan to do other things if the teacher never accepts your lesson.
Last week I even had a teacher stand me up TWICE in the same week. Needless to say I was quite annoyed that she wasted my time not once, but twice in the same week.