More than 50 companies including pharmaceutical giants Pfizer and Merck & Co. were hit with a flurry of bad trades on Monday that later had to be canceled.

Most of the trades that were canceled involved companies in the healthcare industry. Many trades occurred at prices far from their closing price. NYSE Arca and Nasdaq decided to cancel all trades 30 percent away from the prior consolidated price, the exchanges said.

The trades were made in a purchase of an entire portfolio of shares that were sold together, like in a basket, according to a source familiar with the matter.

This is the latest example of a trade execution gone haywire that has come to light since the "flash crash" last May called the public's attention to erroneous trades.

About three dozen trades were canceled in Pfizer at prices that ranged from $27.60 to as high as $88.71. The other companies affected also saw trades far from their normal trading ranges between 4:57 p.m. and 5:05 p.m. EDT.

Other stocks that saw trades busted include Aetna Boston Scientific, Johnson & Johnson, UnitedHealth Group and Zimmer Holdings.