Scarlets play Munster in a Heineken Cup match at Thomond Park. Scarlets win the ball from a tackle/ruck and go left to where prop Rhodri Jones and lock Lou Reed are close to each other. The ball goes to Jones but Reed gets ahead of them. Jones drives into Reed's back as Damien Varley, Wian du Preez and Donnacha O’Callaghan close in.

The referee penalises the Scarlets for obstruction.

There is no doubt that Reed got in the way out of clumsiness. He certainly did not intend to clear a path for Jones. If anything he got in Jones's way.

Surely, this is not worth a penalty and there is allowance made in the law for accidental offside, which is what this seemed.

Law 10.1 OBSTRUCTION
(b)  Running in front of a ball-carrier. A player must not intentionally move or stand in front of a team-mate carrying the ball thereby preventing opponents from tackling the current ball carrier or the opportunity to tackle potential ball carriers when they gain possession.
Sanction: Penalty kick
(c)  Blocking the tackler. A player must not intentionally move or stand in a position that prevents an opponent from tackling a ball carrier.
Sanction: Penalty kick

It would seem clear that Reed did not intentionally move in front of Jones.

Law 11.6 ACCIDENTAL OFFSIDE
(a)  When an offside player cannot avoid being touched by the ball or by a team-mate carrying it, the player is accidentally offside. If the player’s team gains no advantage from this, play continues. If the player’s team gains an advantage, a scrum is formed with the opposing team throwing in the ball.

It seems that most of those 'crossing' penalties when teams try 'clever' moves are accidental. The clear case of intentional happens often at kick-offs where a supporting player will move in front opt the catcher and seek to ward off opponents, at kicks where a player moves off his line to get in the way of an on-coming opponent and in a line-out where a player moves to act as a buffer between opponents and the catcher of the ball.

The problem may well lie in the meaning of intentional. Reed ran where he ran of his own accord and volition. In that sense it is intentional though he clearly had no intention of obstructing. It's not easy - as is the case with the deliberate knock-on but the contact between Reed and Jones makes accidental offside the easier - and more sensible - option.