When Claire leaves Lallybrook intending to leave Jamie the hem of her skirt is clean. It continues to be clean as she approaches Jamie. In the next scene her skirt it has a band of mud at the hem. It couldn't have got so dirty in that space of time.
Both Ian and Jamie are seen swimming front crawl, but this stroke wasn't introduced to England until 1873, so it's unlikely they would know it in Scotland in the 1760s.
Just after Laoghaire shows up unannounced at Lallybroch, when Jamie is trying to tell Clarie he only loves her, you can see Jamie's back briefly (at about 26:44), he does not have any scars on his back.