Muscle spasms

[SpencerH]

Dont fret it, its no big deal. Since its a rear deltoid just ease up on working them directly for a while (although you may have overworked it doing a number of things - even squatting). I do some light isolation work on my shoulders (mostly for side and rear delts) but I've always felt that shoulders get so much work from my other heavy lifts that they dont need much more before they become overtrained.

[Koyaanisqatsi]

When I had a similar problem it was a nutritional issue...a multivitamin fixed it. Which was good because mine was on my face, and facial twitches make people start to keep sharp objects away from you.

[TCO]

I would recommend a little bit of banana/salt on meals/water. Your body can sort out the Na/K balance as long as you have excess. If you are running in hot weather, is possible to be a bit salt deprived (although I think this is more likely to give calf cramps, especially at night).

You could also change your lifts around to not hit it so hard.

[DirtyMartini]

Rest. Hydrating couldn't hurt. I'm a little dubious that a true, measurable nutritional deficiency is the culprit, but taking a multivitamin is generally a good idea, so that couldn't hurt either.

I generally get them when I've been lazy, then get back in the gym or do something else strenuous -- like you said. I generally work out through them and they go away. However, I've never had days worth of twitching like Spencer -- I can see where that would be more than a bit annoying.