If a (specific) minecart is on a detector rail, it does not output a signal when it should. If the comparator is updated (right clicked), the correct signal will be displayed. I have attatched screenshots demonstrating this, using a valid command (/say) and then an invalid (/sa)
NOTE: Where I encountered the bug was while using a command block minecart in the nether, i have not tested it any further than this. I am assuming it does the same for hopper minecarts and chest, and in the overworked, but I may be wrong.
Is this still an issue in the current Minecraft Snapshot 15w47c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.