Right, I meant to blog about this quite a long time ago, but I forgot.
There's sort of a bit of an enormous plot hole in the Doctor Who episode "The Beast Below". Fair warning: There are spoilers regarding this episode of Doctor Who. Also, I'll probably talk as though you've already seen the episode, so it would be good for you to watch "The Beast Below" before reading this, if you haven't already.
So, at the end of "The Beast Below", it is discovered that the titular character (the beast below, not The Doctor), who has been propelling the giant city-ship up until this point, goes faster when it's not being shocked in the brain every few seconds. Specifically, the creature came to Earth in order to help the humans, though it couldn't communicate with them, which is supposed to have allowed this whole scenario to emerge.
To make sure that we're on the same page here: for literally the entire journey of the vessel (a few centuries in length), the beast below the city has been being shocked, in the brain, at very regular intervals. This is where I see an enormous plot hole. It means the following:
-Whoever was in charge of this project had to, somehow, decide to start shocking the alien in the brain once every few seconds before they left Earth orbit.
-Whoever was in charge of this project had to then implement this shocking plan, including making the brain-hole and constructing and activating the probe, again before leaving Earth orbit.
-The probe had to go the entire trip without breaking, needing replaced, or needing to be turned off to be cleaned, repaired, or updated.
-For the entire trip, the probe's power supply had to be uninterrupted.
-They had to never want to stop or slow down. Since they thought that the frequent shocks were keeping the beast moving, they would have reduced or stopped them to slow down or stop.
-Nobody ever said "Hey, what do you suppose would happen if we stopped shocking the creature for a few minutes?" - at least not in a convincing way.
-Nobody ever accidentally turned the device off.
-No scientist in the control room ever went rogue enough to sabotage the shock system. Also, nobody ever successfully broke into the control room or hacked into the controls to do the same.
Basically, the decision to set up this shock system was extremely unscientific - grounded only in ... well, absolutely nothing - and keeping it in perpetual operation would be a logistical nightmare - as anyone who has to maintain any system can tell you.
P.S.: I'd have given more/better details, but I'm going on memory, and the episode was quite a while ago. Like I said, I meant to blog about this and forgot.