Their quality is much worse than vanilla and TD heads.

So while in the Koranyon, Upper level, if you go through the Sturdy Wooden Door which leads to Mount Sarrunabi, if you equip a custom spell, it simply crashes. With normal spells it did not crash, only some custom spells. Simply equipping the spell or trying to cast the spell, it freezes and crashes.

Additional info:
Using OpenMW 0.49

The custom spell that I used to cause the crash:

Bad tapestry bleed in fireplace. Andothren, Zalan's Manor

If you complete the Thieves' Guild questline and oust the Cammona Tong from Andothren before completing the Certificate of Credibility quest, Vermethys apparently disappears from the Andothren Council Club and the quest cannot be completed.

Once the player has accepted the Morag Tong writ for Tolmse Indarys in Balititashpi (TR_m4_MT_Tolmse), Tolmse Adrend in the Old Town section of Andothren will say "You dare desecrate the Madgod's dominion? Death! Death to you!" and force initiate combat the next time she is spoken to. Likely that she has been mixed up/misclicked instead of Tolmse Indarys, who is directly below her in the list of m4 NPCs.