When you place down a minecart, remove the rail, then place a detector rail underneath the minecart without moving it will cause the detector rail to turn on, but not power anything whatsoever.

Steps to reproduce

1. Place a detector rail.
2. Place redstone dust or a redstone lamp next to the detector rail.
3. Place a minecart on the detector rail.
4. Break the detector rail.
5. Place a new detector rail under the minecart.

Expected result

The dust/lamp lights after step 3 and again after step 5.

Actual result

The dust/lamp lights after step 3, but does not light after step 5.