QUOTE (Dgital Grimoire Pg

Focus Addiction (p. 26, Street Magic) uses the Addiction
rules (See Addiction quality, p. 80 and Substance Abuse p. 247,
Shadowrun, Fourth Edition). Th e gamemaster can call for a Focus
Addiction Test at any time she feels the player is abusing foci;
Your example probably wouldn't be an issue, unless you had a very high powered foci, or a very low Magic Rating. But that also depends on your GM. It's obviously pretty subjective, but at the end of the day, it's a GMs call on what 'abusing' is. Armor spells stacked on top of regular armor can be a pain in the ass for a GM without an open mind. But also keep in mind that the armor spell glows. That's something that comes up fairly regularly in discussion.
Older editions were a lot more stringent on this rule, and it could actually affect your ability to use Magic. Now the Addiction hits you with mental flaws instead. I believe the
intent was to curtail over reliance on power foci, spell casting foci, or a "crap ton" (technical term) of sustaining foci, meaning that the mage was leaning too much on their magicky-gadgets instead of their raw ability. Which usually translates into thorough planning and a lot of investments of karma into foci, and now the mage has become a pain in the ass. All well and good for the player of the mage until the GM turns around and pulls the same trick with an NPC. Now
everyone hates the mage.