My use of UML for reverse engineering the existing code perhaps the same amount as my use of it for designing codes to be written. They both server the same purpose though, to give me insights on what to do next on the code. On reverse engineering case, it sometimes reveal certain connection that does not look right. There is a term Code Smell, so perhaps this could be named Diagram Smell.
The same as Code Smell where you see/feel something unpleasant on the code e.g: conditionals that looks like stairs, duplications everywhere, diagrams that is not pleasant to see and draw could hint part of the system that is not optimally structured.
Below is an example of diagram that looks suspicious :
Sometime, when I encounter the construct as above, there is a superclass that is waiting to be extracted. The code can then refactored that when reflected to diagram can be seen as below :
Bad-looking diagram does not always means bad code though. It just a hint for us to look more at the code in case there is potential problem to be fixed. It could be the code is already in good condition while we draw diagram from the wrong understanding of it.