If you add a modifier to a $skiller who doesn't have the $skillname in the skillvals field, it is supposed to do nothing. However, when I double-checked the code I noticed it was comparing the wrong variable. After manipulating skillvals to read a $valstring from the $skillname, it should have checked if $valstring was null then goto block ##addmoderr. But it was comparing $skillvals instead. I just changed it, so hopefully it should reject modifiers to skills the skiller doesn't have correctly now. The code for this is in block ##addmod in object 58416.
Hope that helps.