Untyped properties like the one below makes #develop crash completely with an unhandled stack overflow exception. #d gives little or no additional info.
On my system, the VS debug dialog box is shown. #d continues to work until the dialog is closed. It's hard to tell exactly under which circumstances the error is is trigged. #d may not crash just after adding an untyped property, but opening an existing source file with untyped properties usually or always chrashes #d.
Declaring a as object may be a workaround, but I haven't tested that, or behaviour with duck typing, thoroughly.