The Cross of Satan
An "inverted" latin cross is called St. Peter's cross. When the apostle was to be crucified, he asked, the legend claims, to be hung upside down. Whether this is true or not is, of course, irrelevant; what matters is that the supposedly satanic symbol have been used by the catholic church for a very long time.
Satanists are not the brightest folks to begin with, but you would think they would check to see if a symbol already had a meaning before adopting it as their own. Satanists think if they invert a symbol, it will make it evil or if they spell something backwards it makes it cool. The fact is that this symbol has been known as Peter's Cross for the better part of two-thousand years. It is written and widely believed that Peter was crucified upside down by his own request.
I beseech you the executioners, crucify me thus, with the head downward and not otherwise: and the reason wherefore, I will tell unto them that hear.
The Acts of Peter, XXXVII
Apparently, quite a few people, christians and satanists and "satanists" alike, not to mention everybody else, believe that a symbol is satanic just because someone says so. Perhaps they should check with the good christians below? There are more out there, these are just some I found available on the net. As for the satanists: With so many truly non-christian symbols available, why on earth should you choose a christian one?
Below left: In March 2000, the pope visited a certain holy site. (Trivia: The word cathedral originates from the very chair, cathedra, the bishop used in his church. Since Peter was (according to legend) bishop of Rome, i.e. pope, he's sometimes referred to as Petrus cathedraticus, "Peter in the chair". The use of Peter's cross on a chair thus has quite a symbolic meaning, especially when it's used by the pope. I'm sure this connotation did not escape the Vatican people.)
Below right: The crucifiction of Peter, painted by Michelangelo in the early 16th century.
Related: The Peace Symbol has nothing to do with either St. Peter or Satan.
Thanks to tyskaorden of Skalman for the heraldica