In the beta version of activity diagrams the combination of a backward reference and swim lanes does not work.
This is the combination that produces a broken diagram (the backward node flows somewhere in the middle):
@startuml
|Swimlane1|
start
repeat :foo as starting label;
:read data;
:generate diagrams;
|Swimlane2|
backward:This is backward;
repeat while (more data?)
|Swimlane1|
stop
@enduml
This is a working diagram (swim lanes only):
@startuml
|Swimlane1|
start
repeat :foo as starting label;
:read data;
|Swimlane2|
:generate diagrams;
repeat while (more data?)
|Swimlane1|
stop
@enduml
This is a working diagram (backward only):
@startuml
|Swimlane1|
start
repeat :foo as starting label;
:read data;
:generate diagrams;
backward:This is backward;
repeat while (more data?)
|Swimlane1|
stop
@enduml