Quote: --- Original message by: Snowman_AM2
You don't have to abandon your map, you just need to fix it.

To avoid getting this error, or to troubleshoot it WITHOUT importing the .wrl, try this:
1) Open your map in Gmax.
2) Select your map with the edit mesh-> edge modifier (yes, the whole map).
3) Press f2 and f4 to show highlighted faces and edges (if you haven't already).
3) Scroll in the menu below the modifier stack to "Show Open Edges" or whatever it's called.
4) Check all vertices near these open edges. Weld as necessary.

Quote: --- Original message by: Snowman_AM2
When I say "weld" I mean you select the vertex and use the "Target Weld" on the toolbar beneath the modifier stack. You should use this every time you want to connect meshes or vertices. Gmax tuts will help you out if you don't know how.

Pressing f2 and f4 just shows faces and edges in red when you have them selected. You can press either at any time - they just toggle the modes. Personally, I could not imagine a worse kind of hell than working with Gmax without these both on.

Regardless, you'll still want to use the .wrl importer in the future, or you might need it now - "select open edges" only does so much.
1) Follow the directions in the importer's readme to install the script in Gmax.
2) Open Gmax.
3) Click your new "Import WRL" button.
4) Navigate to your Halo Custom Edition directory.
5) Open the .wrl that corresponds to your map.