Plantuml requires the diagram to start with @startuml tag and end with @enduml tag.
Problem is that when user forgot to put the @startuml tag or made a misprint, plantuml just ignores the file and returns no result. That is a problem not only because I lost few hours trying to figure out on the hard week what's wrong with my parameters to planuml command : )
We have a console tool that uses plantuml to generate images for quite a lot of diagrams. If there's an error in the diagram source — it's perfectly fine, plantuml returns an exception and we can inform the user about it. If the diagram source file lacks @startuml tag — the response from plantuml is the same as if the diagram was built correctly (e.g. no response at all). User is not informed and he thinks that all of his diagrams were fine which is not true. Now we have a workaround: check that the image was actually created. But that's not very beautiful.
SUGGESTION: reutrn an error or a warning if no diagrams were found in the file pointed to plantuml.