If you place an empty minecart of any type on a detector rail, and the place a comparator directly adjacent to the rail, so that it is facing away from the rail, then SOMETIMES (but not always), the comparator will be activated when placed for a short pulse. The pulse is long enough to invert a redstone torch. I have no idea what causes this bug, and so far I've only seen it happen with detector rails, but if anyone finds similar behavior happening with comparators, please let me know in the comments.
See this video for a demonstration of the bug: https://youtu.be/tX6iSmdMCbY?t=46s

Steps to reproduce:

1. Place a detector rail and place an empty minecart on top of it.
2. Place a comparator adjacent to the detector rail so that it is facing away from it.
3. If you don't see the comparator blink on for a moment, break it and place it again. Repeat multiple times, and you should see it give off a short pulse on one of the tries.