@startuml
!pragma teoz true
A -> B : something
B -> C : something else
C --> B : something \n more
B --> A : abcd
||40||
A -> B : something
& B -> C : something else
C --> B : something \n more
& B --> A : abcd
@enduml
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuL8iA4fCpqrKA4dDhr8eAIfDvN9KqBLJS5AmKYZEpqqjoSZCI-To0Wav8miffEOKft8vA-YYgrM8oLF8pIzA1Ua1oZa2vHAJabEuk6fgJ0ngQhYm6Q-cG8u5O5t8LZYavgK0lGG0
The last two messages should be at the "same time" but `abcd` is earlier than `something more`.