A lot of times (not always) when I'm close to finishing the number of items due for the day, based on the schedule that Skitter makes for me, the number of reviews stops going down and actually starts going back up. Just now, I got down to 13 and I was getting item after item right, but nothing was happening to the number of items to review. Then it actually started steadily going up, eventhough I wasn't getting anything wrong.
I guess I don't like this because I think it defeats the whole purpose of having the Practice page displayed the way it is,with number of items due per day, etc.
Another thing is that some days the queue will actually go to "zero items due", but an hour later there will be 15 items due, even though I completed the number of items due and did a few extra.

To me, this all sort of makes the review queue less reliable because I don't know if the number of items due "are really due", or if I'm being over worked

If I've been practicing for awhile and the queue is lingering around 10-20 items I usually refresh the page. When the page reloads it's usually at either 0 or 1.

Yeah, for me that number jumps around all over the place, up and down by 20 or so sometimes.

I go by the readyness % of the words.

Unfortunately, the review bar number gets a little unreliable just as you're getting down toward 0, because the state of all your items as the Flash client knows them gets a little out of sync with the server. So getting down toward 0 and starting to see items with a readiness less than 100% is a better indicator.

The items that were due an hour later are probably ones you just got wrong during your practice session or were becoming due as shown on the future items graph on the practice nav page.

@Nick

I don't know if this will help you hunt a bug, or if it is even a bug, but I have noticed something. As I am working away I have my queue first goes up for a while and then does a massive drop a bit down the road. It is as if the system is accidentally adding to the queue instead of subtracting for a time and then corrects it.

For example: I start at 300 items and am working away. It drops down normally to 275 incrementally. This is how it is supposed to work. However, it then shoots back to 300. I keep working a while and then it suddenly drops down to 250.

Yeah, I got this bug on file. I am not planning to work on it soon, though. The current code is nearing complexity limits and needs some heavy refactoring if it's ever to work with total accuracy.