Excessively dangerous weapons are weapons which have been deemed too dangerous for competition.
Dragonstrike was removed from BB# after a piece of their blade broke off and became embedded in the arena glass. The arena was not breached but it was deemed that that robot had too much potential for danger to continue in the competition. They were removed with honorary mentions as they were unable to complete the competition.
Megabyte was deemed to have an Excessively dangerous weapon in robot wars 7 (citation needed).