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Probably what is being referred to is the danger that occurs when two electrodes come together that are charged with different polarities... When they contact it will double the voltage..
AC will bring that situation about easily if it comes into contact with another electrode with DC+ DC- or another AC electrode.
See the American National Safety Standard
Z49.1 Safety in Welding, Cutting, and Allied Processes E11.3.6.3
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http://www.aws.org/pr/jun17-2005.html