Stacked minecarts go forever if stacked on eachother. Originally found with chest minecarts and powered rails. I didn't found any of this reported yet so... I used a modded client but the bug is also in vanilla (tested). I attached a video with this bug it was too large, so i uploaded it on my website: http://tonkunservu.netau.net/Files/bugs/bug/bug.html
If the video stops during playback, it might still be uploading.