But the 33-year-old record Welsh try scorer recovered just in time to line up in a back three which also features Scarlets' Morgan Stoddart and Perpignan-bound James Hook.
And as he prepares to face England for the eighth time in the Championship, Williams is on a mission to add to his one try scored against Wales' bitterest of rivals.
"England seems to be the special one, not only for the players, but more so for the supporters and the country," he said.
"There's certainly an edge of excitement in the air at the moment.
"I put myself under a lot of pressure every time I play, but perhaps a little bit more this time.
"I'd love a very successful year with Wales, winning the Six Nations and doing well in the World Cup and that will be it for me really.
"I can look back and say I've gone out on a high, thoroughly enjoyed the experience, and then let the likes of Leigh Halfpenny, George North and Tom Prydie enjoy their time."