Since there are so many freqeuentally asked questions about the same stuff that some people figured out befoure or that already faced it woudl be nice to have an organized thread with all the bigs , problems , error types in with the proper solution next ....
So far the first simple ones that come to my mind :
Exporting from 3dsmax to nif format ( first of follow the tutorial for it with the proper nif plugin settings )
1 The mesh ingame has some spiky points when animated -------------------- wrong vertex rig skinning , fix it in the skin modifier
2 the exporter crashes your 3dsmax when exporting --------------------means you didn't give the BSdismemberment modifier a proper selection
3 Freeze ingame when loading the mesh -------------------- probably you may have a doubled mesh file inside the nif
4 Instant crash of the game when you loot some replaced item -------------------- check the naiming of the mesh inside the nif
5 weird shadowing on the mesh when looking at it ingame on outside -------------------- reduce the amount of bones affected by rigging from 20 to 4 and remove zero vertices with a % less than a 0.something of your choice
6 the morphing isn't working in between phases -------------------- you need to modify the _1 or _0 version to adapt to the _0 or _1 versions without altering anything , deleting polygons or adding more , just move vertices around or rotate but no add or remove
7 the mesh imported in max and reexported with a minor tweak that didn't alter the rig , same for the _0 and _1 but the morphing is messed -------------------- if the meshes are imported from the game and not altered and reexported , you need to make a new polygon by scratch , convert to poly , add the mesh to this and reexport , then make the _0 or the _1 out of this
8 the somuchmonster tool for making the morphed shape results in a messy or not really good looking shrinkage of the mesh I wanted to produce -------------------- use a lattice modifier under the stack of skin and on vertices to produce a new mesh shape
9 There is a big triangle ingame in place of my mesh -------------------- you have forgot to remove some unneded materials in the nif and the game can't sort well the material
10 my mesh is invisible -------------------- check the materials if are correctly allocated in first property the texture and second the alpha property , the texture may need an alpha
15 : I have some random transparent faces on my mesh after exporting from 3ds max ----------------- you may have some polygons that are not assigned to a specific type under the dismemberment modifier in 3dsmax .
Add on by Goghiel
10a: My mesh is invisible -------------------- if it is only invisible when equiping other items like boots and gloves, it's because the wrong Body Part Partition is assigned to that face selection.
11: Insta crash when equiping my custom armor -------------- Check the Body Part Partition names, if it using something other than ones recognised by the game engine, like the BP_Torso as used in F3, and not something like "32"
12:My meshes normal map doesn't seem to render at all even though it's linked into the correct BSTextureSet slot -------------- In block details for the NiTriShapeData make sure Has Normals is set to Yes. Num UV sets is set to 4097. And then Spells>batch>update tangent space.
13: My mesh is black/really dark. ----------------- Do the above fix, but this is likely to do with your shader type, and the shader flags. If it is using the environment map shader make sure that flag is set, and there is infact an environment map set up in the correct slot. Other wise use the normal shader type. And also the shader flag for SF_Vertex_Color and the mesh does in fact have vertex color.
14: I have some random black faces on my mesh after exporting from 3ds max ----------------- The exporter has some issues with exporting Binormals and Tangents, spells>batch>update tangent space.
well those are some of the most common I have run into and may be could be of help to someone ....
This is not a tutorial so if you are not understanding something look for the appropriate tutorial , if you also have some other issues fixes please list so we can add ... if you fid something isn't quite correct please say and we can fix....

