The Washington Wizards ultimately traded Jordan Crawford to the Boston Celtics for Jason Collins and Leandro Barbosa, but if it were up to them, they would have taken Chris Wilcox instead. However, Wilcox apparently didn't want to come to D.C.
Jason Collins wasn't supposed to be in original deal but sources say Chris Wilcox refused to waive his Bird rights & forced BOS to try again— Michael Lee (@MrMichaelLee) February 22, 2013
Wilcox is one of several players on his second consecutive one-year minimim-salary contract. Due to a salary-cap quirk, such players lose their Bird Rights in any trade, so they have veto power over any deal. The clause is colloquially known as the Devean George rule, because the little-used Mavericks forward famously used the clause to hold up a trade that would have sent Jason Kidd to Dallas several years ago. Three years ago, James Singleton had the opportunity to use his veto power to hold up the trade that sent Caron Butler to Dallas, but decided to allow the trade to be completed.
The trade needed one more minimum-salary player because a straight up swap of Crawford for Barbosa would not have worked salary-wise. Therefore, the Celtics were forced to throw in Collins.