Helicopter - if you weight the front slightly with blu-tak or gaffer tape you might have more success, but they are flighty little beasts doomed to whizz round crazily. You can now buy competitive ones which fire lazers and can shoot each other out of the sky!

Watership Down - the first copy of the book I had had a big OS type map in the cover of Watership Down and all the places around it, so I never doubted it was real.

It actually came with little sticky weights you could attach to the nose, and we did try them; but the trouble with them is that (a) the clever auto-stabilising mechanism helpfully compensated for them to a large extent, and (b) they add to the total mass of the thing, which you don't want because it takes more energy to keep it in the air and drains the battery faster.

A better solution seemed to be to fiddle with the tail fin so that it deflects the blast from the tail rotor downwards, hence pushing the tail up and the nose down.

Additionally, the docs reckon you get a bit of forward thrust when you turn it in the direction requiring more tail-rotor power, presumably because the deflected blast as per the previous paragraph is greater, so the tail tips up more. So they recommend driving it in long curves. Sneaky.

(I suppose, though, that it really would have been too mechanically difficult to put a cyclic control on the thing. To say nothing of the increased pilot skill required.)

In addition to mechanical difficulties, the other traditional limit has been number of radio channels, I think [although I see that Wikipedia says this one is infrared anyway]. (It occurs to me that you ought these days to be able to digitally stuff multiple inputs down one channel rather than needing a channel per analogue input -- does anybody do that?)