When a minecart is stopped on a slope over a detector rail, no redstone signal is output. As opposed to 1.7.4 (screenshot included in attachments for comparison), where the minecart is detected. I have only seen this in 14w06b , though I have not checked dev versions prior to it. Let me know if this is truly a bug, or an intentional change in the way detector rails function (or if someone else beat me to reporting this bug so I can check out that post instead).

To reproduce:
1. Create a slope with rails. On of the middle rails should be a detector rail
2. Place a block so that the minecart will not fall down the slope
3. Place a minecart and observe the lack of redstone output