Product review (1 stars): they replaced the cooktop mirror part and said they never had any problems like that before (I don't believe that)
I have the 30" thermador mirrored finish Induction cooktop.  It's gorgeous.  But that's about it.  I bought it last May, and one day one of the burners stopped working.  Then I noticed tiny black spots under the mirror part.  Every time I used it, more black spots came.  All around the edge.  they replaced the cooktop mirror part and said they never had any problems like that before (I don't believe that).  Then the spots tarted happening on the replacement.  Then they replaced the entire cooktop after trying and failing to fix the burners.  The new cooktop is about a month old and I just noticed black spots. I'm about done.