This is a project I have been working on for some time and the servo WAS working. I had to set it aside for some weeks, came back, removed one of the two servos and replaced it with a 28BYJ-48. (Couldn't get the servo to run slow enough).
Now back on the project and other parts are working fine. I have a scope on pin 10, and when after boot, i get the correct servo signal whose width varies as it should when moving a control.
When I plug in the servo, the signal on the scope drops instantly to around 1 volt or less! Have tried another servo - same same.
I did have to add the 4 coil wires for the 28BYJ and have combed the board for bridges, etc, and all looks good. Just when I connect the servo the voltage is pulled down - and sometimes shuts the whole system down! When I hit reset it always comes back to life... so AfAIK nothing is fried.
Since this was all working - now wondering if the Arduino Pro Mini has died/partially failed. Maybe the current draw is now too much for it? Am running the 5V on an external supply, all grounds common.
Have scoured the board visually and resistance checks and everything comes up ok... So now I'm thinking I may to replace Pro Mini - sure hope not! 
Ideas?