If you use bone constraints and delete a bone that was created BEFORE the constraint target bone all of the constrained bones "shift" the selected bone for contraint to to the next one in the list.
This depends on the order the bones were created. If all the constrained bone targets are at the top of the list (created first) than bones deleted under them will not effect the constraints.
Constraining is apparently linked to the internal bone list rather than to the actual name of the bone. So when a named bone in the list is deleted the constraint targets "move" up a notch up messing up the constraints.
-Vern
Bone constraints target shifted when other bones deleted
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...BTW! 
Another related thing talking about constraints is that if you select, or have selected, several bones meantime the Bone Constraints pop-up window is opened, the Angle Constraints and Dynamic values be copied to all that selected bones, and it can be specially irritating working with Dynamics cause this window must be opened to can animate it along the time... CIAO! 
EDIT1: Hmmm, I don't know, but maybe even a good scripter could find an easy solution for this issue...
EDIT2: ...Or even a not good scripter like me taking out of the Window that "Bone dynamics:" check box as a provisional mend... Hmmm... I'll see...
Another related thing talking about constraints is that if you select, or have selected, several bones meantime the Bone Constraints pop-up window is opened, the Angle Constraints and Dynamic values be copied to all that selected bones, and it can be specially irritating working with Dynamics cause this window must be opened to can animate it along the time... CIAO! EDIT1: Hmmm, I don't know, but maybe even a good scripter could find an easy solution for this issue...
EDIT2: ...Or even a not good scripter like me taking out of the Window that "Bone dynamics:" check box as a provisional mend... Hmmm... I'll see...