Before we get in to the limitation, here's a little context on what's happening server-side.
In Office Web Apps Server 2013, documents are converted server-side before they are rendered in the browser for editing.
During the conversion process, the document is ripped apart in to mostly .png and .xml files and cached on the Office Web Apps server.
This means that when you are editing a document, you're not actually editing a .xlsx or .pptx, etc. file. You're editing the converted and cached file.
In PowerPoint, you can add objects to slides called SmartArt. SmartArt includes many different options, but the one we'll be focusing on is the Organization Chart.
When you use the Organization Chart you can add objects above or below the currently selected object by right-clicking a shape.
In Office Web Apps Server 2013, the server will not be able to convert the file server-side if the hierarchy is more than 9 levels deep.
Looking at the screenshot below, if we were to add one more level to this hierarchy, the document would fail to load in the browser for editing and we would find conversion failures in the Office Web Apps Server ULS logs.
To avoid this issue, do not create SmartArt object hierarchies more than 9 levels deep.