On the subject of tangled bloodlines, we have this curiosity:

In 1420, during the Hundred Years War, a settlement between the French king Charles VI (or his agents; Charles was mad again at this time) and Henry V made Henry the de facto ruler of France. The English king took Charles' daughter Catherine as his queen, and became Heir to the French Throne and Regent of France.

Henry's line was extinguished in the Wars of the Roses.

After Henry's death, the queen began an affair with her Keeper of the Wardrobe. This man held no social rank, so the liaison was particularly scandalous. This man's name was Owen Tewdwr. Catherine and Owen's oldest son Edmund died in 1456, leaving behind his pregnant wife, the 13 year-old Margaret Beaufort. The child was born in 1457 and eventually crowned Henry VII in 1485 after defeating Richard III at Bosworth Field.

So, though one would have assumed in 1420 that Henry V's descendents would rule France, it was Charles VI's great-grandson who ruled England. His great-great grandson and great-great-great granddaughter became two of England's most famous monarchs, Henry VIII and Elizabeth I.