I use Fireworks for creating wireframes. One FW png file may contain many layers
with each layer representing a web page. The intention is to create the png file, and then to export it to a PDF file so that developers can use it to view the pages and to understand page navigation. I learned this technique by watching Jim Babbage's excellent video on using Fireworks for wireframing.
I create hotspots in the FW png file on one layer and use that to point to another layer that represents another web page. The purpose is to show a kind of flow or extra info for the developers. I've been struggling for over a year with an error that would pop up now and then when attempting to save the png file. The error would show "Internal Error Occurred" and I had no idea why this was occurring. I tried multiple things to get rid of the error and recently I finally discovered what was causing it.
It turns out that if you create a FW png file with layers, and you insert a hotspot that points to one of those layers, and then later when editing the FW png file you decide to delete a layer that is being referenced via a hotspot in a different layer, this error will occur. It doesn't occur every time you attempt to save the png file, but it will occur if you save the png file (no error shown), then export to a pdf file, then (as required by FW) attempt to save the png file again. That's when you will see the error for sure.
The cause is a hotspot link that points to a layer that no longer exists. This is most often caused by creating hotspots that point to the layer, then later deciding to delete that layer but you don't go back and remove any hotspot links to the deleted layer. For some reason, FW will let you attempt to save the file and may or many not show the dreaded "Internal Error Occurred" pop-up message. If you were able to save your png file and didn't see the error show up, and then you decide to export the png file to a PDF file, FW requires you to save the png file after the export option completes. It's when you attempt to save it at this time that it will show up for sure if your hotspot points to a deleted layer. The error message won't go away either.
It's kind of a pain if you have a bunch of layers with a bunch of hotspot links, but you will have to click on each hotspot and determine if it points to a layer that exists in the current png file. If that layer doesn't exist, you will have to point to a layer that does exist, or delete the hotspot to get rid of the "Internal Error Occurred" message.
Hope this helps,