Mental Misstep

• WWizards of the Coast2011-06-01
  Official Ruling
  Each Phyrexian mana symbol in a spell's mana cost contributes 1 to that spell's mana value. For example, Mental Misstep's mana value is 1, regardless of how its cost was paid.
• WWizards of the Coast2011-06-01
  Official Ruling
  To calculate the mana value of a card with Phyrexian mana symbols in its cost, count each Phyrexian mana symbol as 1.
• WWizards of the Coast2011-06-01
  Official Ruling
  If a spell has in its mana cost, X is considered to be the value chosen for it while that spell is on the stack.
• WWizards of the Coast2011-06-01
  Official Ruling
  As you cast a spell or activate an activated ability with one or more Phyrexian mana symbols in its cost, you choose how to pay for each Phyrexian mana symbol at the same time you would choose modes or choose a value for X.
• WWizards of the Coast2011-06-01
  Official Ruling
  A card with Phyrexian mana symbols in its mana cost is each color that appears in that mana cost, regardless of how that cost may have been paid.
• WWizards of the Coast2011-06-01
  Official Ruling
  If you're at 1 life or less, you can't pay 2 life.
• WWizards of the Coast2011-06-01
  Official Ruling
  Phyrexian mana is not a new color. Players can't produce Phyrexian mana.

Rulings via Scryfall

• WO
  Wizards of the Coast2015-02-03

  Mental Misstep was banned in modern on February 3, 2015. Modern kicked off with a banned list that was created in large part based on Legacy play, the old Extended format, and decks played at the Community Cup that year. We are going to allow turn-four combination decks, but not decks that consistently win the game on turn three. Source: https://magic.wizards.com/en/news/arcana/brief-history-modern-banned-list-2015-02-03
• WO
  Wizards of the Coast2019-08-26

  Mental Misstep was restricted in vintage on August 26, 2019. Mental Misstep has been a controversial card among the Vintage community for years. While it does provide more opportunity for interaction, it is also at its strongest against the most interactive decks. We believe that restricting Mental Misstep will open up more deck-building diversity, strengthen interactive decks' matchups against Shops, and weaken Dredge. Source: https://magic.wizards.com/en/news/news/august-26-2019-banned-and-restricted-announcement-2019-08-26
• WO
  Wizards of the Coast2011-09-20

  Mental Misstep was banned in legacy on September 20, 2011. Looking at high-level tournaments, instead of results having blue and nonblue decks playing Mental Misstep, there are more blue decks than ever. The DCI is banning Mental Misstep, with the hopes of restoring the more diverse metagame that existed prior to the printing of Mental Misstep. Source: https://magic.wizards.com/en/news/feature/explanation-september-2011-br-changes-2011-09-20-0
• WO
  Wizards of the Coast2011-09-20

  Mental Misstep was banned in extended on September 20, 2011. To avoid repeating this, the DCI is banning the cards that have been most played in the most successful Standard and Extended decks over the past three years. For reasons similar to why these cards were banned in other formats, the DCI is banning them in Extended. Source: https://magic.wizards.com/en/news/feature/explanation-september-2011-br-changes-2011-09-20-0