I've come across a bug related to player units, skills and network play.
In my mod Classy Classes I've added all the skills from all the classes to the all the base units for all classes. Resulting in each class having access to all skills. It's a pretty simple mod, but it adds a lot of skills to the BASE .DAT file.
The mod works fine when you play single player, but if you try to play in LAN, (and most likely over Internet, I've not tested this) the game will crash as soon as the connecting player is about to enter the game. You can hear the ambient sounds playing, but the game hangs at the load screen and crashes after a few seconds.
After hours of testing I've figured out what's causing the game to crash when trying to play in network. It seems the game can't handle pulling all the skills from four different directories into one player unit when it's connecting via the network.
There is one exception to this, and it's the Outlander. If i load the skills from the other classes in a particular order, I can make a clone of the Outlander come through the network without crashing the game. The class skills most be loaded in reversed alphabetical order in the .dat file: Wanderer, Railman, Berserker, Arbiter. For other classes I can only manage to load one additional class's skills before it crashes. I tried loading the other class skills in all possible combinations on a Berserker and it simply will not work. I've only done extensive testing with a cloned Berserker, but I think the game handles the Outlander differently in the game code. Like it's the first character they made, and it's the base for all others. IDK. This just speculation...
The only way I can see around this issue, is to clone all the skills and place them in the same directory, then assign them all to the player units. I don't know if this will work. There might be a limited number of skills you can assign to one player unit, but I have a feeling it's has to do with the directories where the skill files are located too.
Edit: I've reported this on the Runic boards. It has some more details:
http://forums.runicgames.com/viewtopic.php?f=54&t=60119