Situation:
Two concurrent transactions add a new variable to an execution.
In this case, optimistic locking does not work since the row is freshly inserted. (cannot lock on non-existing row)
As a result we end up with two rows which have the same execution id and the same name.
When deleting the execution, only one of the rows is deleted which causes a Foreign Key constrain violation since the second row still references the Execution.

Could be fixed with an integety constraint on EXECUTION_ID and NAME_ as well es TASK_ID_ and NAME_

Failing testcase:
https://github.com/camunda/camunda-bpm-platform/commit/6fffa99cbfe91204f402174349a8f1c87347efe1

See also:
https://groups.google.com/forum/?fromgroups#!topic/camunda-bpm-dev/WPHPzBCsTL0