What version of LabVIEW are you using? What are you trying to compile? Can you create and post a simple VI that demonstrates this error?
I found the error. I had copied most of the VI from a similar one. The VI consists of a loop for maintaining an array of objects representing power supplies, which are in a different class hierarchy from the VI I copied the block diagram from (which dealt with oscilloscopes). There was one sub VI that I had forgotten to replace with its equivalent from the power supply classes, so I had a VI loading a scope from a file and sticking it into an array of power supplies. I don't know if it's a bug or a known limitation of LabVIEW's type system, but the wire from this sub VI to the "insert into array" function was not shown as broken; instead, I got the above compile error.
I'll see if I can reproduce the problem and post a simple example VI here, though it might be hard since it depends on having at least two different classes.
Edit: By the way, LabVIEW 8.5.1 on Windows XP.