"Tsubaki/Kobushi Factory remind me of C-ute"
- Do not say this, no matter how casual, or well-meaning it is, it will start a shitstorm. In will come the old guard of Berryz fans "Stop bringing C-ute into this!!!!THEY BERRYZ BABIES!!!", the the C-ute fans emerge to rub salt in the wound "You're just jealous because our group lasted longer LOL Berryz fans are so bitter!!1". By this point, you'll probably notice a whole different breed of Factory fan "Don't compare them to either, let them be their own thing!!!1". You might think this will finally allow the argument to die down but don't be fooled, this will never end.
It may seem like an innocent enough statement but there's a lot of old wounds wrapped up in this mess. It's best to take the route of avoiding mentioning C-ute in any discussion relating to the Factories, lest a shitstorm ensue.
"I hope [member] graduates soon"
- This does not need explanation. Why would you say this? What is wrong with you?
"That KSS is overrated"
- Ahhh no. KSS fans may be small in numbers but they're big in devotion. This is Especially true of members who have been in the program for a long time (more your Ichiokas, Danbaras and Kagas than your Yokoyamas or Kasaharas). After they've been in the group for a few years, the inevitable shitstorm will decrease to the size of any other member but do not say this when a girl has just debuted, or god forbid, when they are still in training. Blood will spill.
"Angerme is better then Morning Musume."
Oh no you Didn't! Never say this. Ever. You will be killed.
Anything about Maachan
Our Sato Masaki is a figure of controversy. Whether you love her or hate her if you wanna live a peaceful life, it's best to keep that to yourself.
Anything about weight, examples: Westerners only liked Zukki because of her weight, Haachin is too thin/fat
1. This is kind of a douche move in general.
2. No matter how respectful or well meaning your comment is, someone will take offense and a shitstorm will ensue. Best to avoid.
Anything at all about the dating rule.
The dating rule is a sore subject to most idol fans (most people will lose a beloved idol to this rule at some point). Furthermore the controversial nature of the "rule" means that mentioning it will almost always result in an argument.
Photobooks and age. In general, sexualisation (or not) of minors.
Like the dating rule, another controversial topic. If you see a photobook announced for a girl under 18, it's best to go into hiding for a bit, regardless of the book's actual content.