Player A activates Simic Guildmage's first ablity and targets two of his own creatures. In response, player B casts an instant to gain control of the second creature. When resolving Simic Guildmage's ability, according to the card rulings, the counter will not be moved.

I guess this is because of CR 608.2b:

(…) However, if any of its targets are illegal, the part of the spell or ability's effect for which it is an illegal target can't perform any actions on that target (…)

On the other hand, CR 121.5 does not cover this case:

If an effect says to “move” a counter, it means to take that counter from the object it's currently on and put it onto a second object. If the first and second objects are the same object, nothing happens. If the first object has no counters, nothing happens; the second object doesn't get a counter put on it. If the second object (or any possible second objects) is no longer in the correct zone when the effect would move the counter, nothing happens; a counter isn't removed from the first object.

I suggest to change the last sentence to

If the counter cannot be put onto the second object (or any possible second objects) (e. g. because it is no longer in the correct zone when the effect would move the counter or has become an illegal target), nothing happens; a counter isn't removed from the first object.

How about trying to move a counter onto Tatterkite? If the counter stays where it was, then the sentence should include that example too:

If the counter cannot be put onto the second object (or any possible second objects) (e. g. because it is no longer in the correct zone when the effect would move the counter, cannot have counters placed on it, or has become an illegal target), nothing happens; a counter isn't removed from the first object.

Can someone please confirm or point to a rule a have overseen?

The Gatherer ruling is correct, if the two creatures are not controlled by the same player at resolution, the second target is illegal, and you can't perform actions on illegal targets.

Moving a counter to or from an object is performing an action on that object. There is no need to specify the rules further - they are sufficiently clear on the result here.

If you're interested on rules wording, templating, and similar considerations, you may want to consider the following forum: http://community.wizards.com/go/forum/view/75842/134782/rules_theory_and_templating

Daniel Kitachewsky
L3, Paris, France
Rules Netrep

Edited Daniel Kitachewsky (March 1, 2013 09:53:18 AM)