Halloween Massacre

Canada's decision to tax all income trusts domiciled in Canada. In October 2006, Canada's minister of finance, Jim Flaherty, announced that all income trusts would be taxed in a similar manner as corporations at a rate over 30% on taxable income, causing unit holders' values to decrease dramatically virtually overnight.
Income trusts, which were permitted to make distributions to unit holders on a pretax basis under old Canadian income tax laws, were a popular investment vehicle in the early 2000s, especially in Canada. The Canadian energy sector was hardest hit by the change, and suffered an estimated loss of about $35 billion to investors, giving rise to the term "massacre".
This change in the Canadian tax law, which was largely debated after the fact, was made to remedy a perceived loss of tax revenue.

Source: http://www.investopedia.com/terms/h/halloween_massacre.asp