This happened at a tournament I was at where someone drew 7 cards after he had already mulliganed. I told him he had to shuffle and draw 5 new cards, in essence a "forced mulligan".
That wasn't entirely correct.
This is a "Game Play Error, Improper Drawing at Start of Game". At regular REL, this results in a caution and a judge will randomly remove the excess card(s) from the player's starting hand and shuffle them back into his library. At competitive REL, this results in a warning and a judge will randomly remove the excess cards plus 1 from the player's starting hand and shuffle them back into his library (pretty much the same thing as a "forced mulligan").
