Consider the following pipeline script.

def str1 = ''
def str2 = ''
(str1, str2) = addStage()

def addStage()
{
    stage('Stage 1')
    {
        node('master')
        {
            sh 'echo run stage'
        }
    }
    
    return ['foo', 'bar']
}

When running the pipeline it will execute the shell command twice. This came unexpected to me and I think it is a bug. When the parameter unpacking is removed from the third line, the script behaves as expected.