Mojira Archive
MC-32972

/summon accepts arguments that it will ignore

/summon "allows" you to have data specified without a position, but does not use that data.

/summon Creeper {powered:1}

will not work, while

/summon Creeper ~ ~ ~ {powered:1}

will work.
(Assuming I didn't misspell the tag; however I have tested it with less obvious tags and it is this case)