jareddm wrote:
Yes, many. In fact, as is mentioned in HH4: Conquest, nearly every legion had marines fighting on both sides or even neither side of the Heresy. Iron Hands, Raven Guard, Space Wolves, and White Scars all have listed examples of marines who either joined with Horus, or went off on their own. And I have no doubt at all that this holds true for every other loyalist legion as well. Given the size and the diversity of each legion, it's perfectly reasonable to say that at least a small handful, especially those who were working alongside traitor legions for long periods of time, fought alongside traitors. Now in a legion of 100,000, a small handful could be 200 and they'd basically never be missed.

The reason for this turning are numerous. Marines have a tremendous spectrum of loyalties to manage and each marine values certain loyalties over others. Loyalty to legion, to primarch, to homeworld, to squad, to immediate superior, to the Imperial Truth, to honor, to their own skill and many others. All of these collide and conflict and depending on what a marine values, they may find themselves on the opposite side of their legion.

jareddm wrote:
Out of game, FW are trying to establish as much precedence for flexibility as possible. The increased legion sizes, the legions themselves having numerous far-flung contingents, the mix of marines from Terra, the homeworld, and neither in each legion, and legions operating alongside each other within expedition fleets all allow for any legion's marine s to find a reason to choose a different side.

As for the examples we do have, there was a Great Company of Space Wolves bearing the Serpents Eye of Horus, who slaughtered millions at Neo Cadiz in 008.M31 as well as Iron Hands who wore a dual Eye of Horus/cog heraldry and who fought at the Siege of Mezoa. There were also Terran Raven Guard who greatly respected Horus above Corax and had begun to join the Word Bearer lodges. While the vast majority of these marines died during the Battle of Gate Forty-Two, there could still be a small smattering of them either amongst the legion or who fled at Istvaan. The FW books make it clear that these are only the most well known cases and they plenty of others existed across all legions, in both directions.

jareddm wrote:
At least one Raven Guard was mentioned as operating alongside a Night Lord contingent, although that might've been a forced conversion.