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A lot of satellites are spin stabilised, where most of the vehicle spins, but the antenna arrays are de-spun to give stable coverage. This can give a regular flash, or variation in visual magnitude.
Many out of control satellites are tumbling, so can give various flash patterns.
The most interesting one I've seen was in the late 80's or early 90's and was a spinning geostationary satellite. I was observing with my trusty (yet very wobbly) Tasco 59T when I noticed a small flashing object in the field of view. I watched it for a while, the object not moving, but the stars passing through the view (fixed alt/az mount). I thought for a while that it might be an aircraft a long way away, but it just stayed right there. I got out the bino's and observed this thing for about an hour moving through the star field, but staying fixed in relation to the earth. In theory it should have been there every night, possibly even still there now, but I never did find it again.
Cheers,
Jason.
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