Q:

Twitter icon upside-down

If I use Twitter icon upside-down as my site favicon, will it be a copyright/trademark violation?
I have copywriting slogan "Down with Twitter" among others on my site.

A:

...will it be a copyright/trademark violation?

You'll find out when Twitter threatens to sue, or does sue you. How much money do you have to defend yourself?
The court (judge or jury) will decide if your usage infringes and dilutes Twitter's business and trademark, or not. Or, if you claim usage of the upside down icon is a parody, the court will decide if indeed your usage could be viewed as a parody by the public.
You might win.

