According to the Arelith Wiki, Shadow Mages get +1 caster level per shadowdancer level. However, this level is not being used for spell penetration. Consequently, a 10 wizard/20 shadowdancer acts as if they only have 10 caster levels to penetrate spell resistance.
Steps to reproduce the Bug:
Create a 10 wizard/20 shadowdancer Shadow Mage on the PGCC with a race that has spell resistance, and attempt to cast a non party safe spell in your area.
Expected Result:
As a 30th level caster, your caster level should beat an SR of 32 most every time.
Actual Result:
100% of spells fail to beat spell resistance.
Screenshot(s)

I believe this bug and the previously reported failure to resist dispel bug viewtopic.php?f=12&t=25281 are due to the class not being included in spell penetration and dispel resist fix that improved palemaster spellcasting several months back.
I am also not sure why two messages of failure to breach spell resistance appear. Perhaps the script is attempting to work and failing?