Halt taking honey in his coffee. The other three recurring Rangers (Will, Gilan and Crowley) make it clear that it's considered incredibly weird, and nobody else does it. But Halt having a private sweet tooth makes sense. He grew up as the crown prince of Clonmel. So, unlike the other protagonists, he would have grown up having sweets on a near daily basis. He's more used to sugar than the other Rangers. What's unpleasantly sweet to them is probably delicious for Halt. That's just how his palate developed.
Furthermore, lots of nobles practice archery when they're young. No wonder he's already an expert shot by the prequels-he's probably been practicing since well before he met Pritchard, who just "tightened up his technique."
Horace's knighting is a much bigger deal than may be immediately apparent to the modern reader. In our world, squires were usually knighted by the knight they served under, meaning that an Araulen apprentice would presumably be knighted by the Battlemaster or one of the Battleschool instructors. Getting knighted by the king is, in and of itself, a huge honour, let alone the fact that he's being knighted early and assigned to the Royal Guard.
On top of that, it also makes sense that he gets the early knighting and promotion, especially compared to Will. Not only does Will have more skills to learn than Horace does (map-reading, navigation, tracking, archery, knife-throwing, stealth, hand-to-hand, and drinking coffee with honey), Will spent the past year traveling with the Skandians, being enslaved, and then recovering. Other than keeping himself in shape physically on Skorghjil, he barely did anything to advance his training, and had to re-learn his archery and knife skills at Hallasholm and work to get himself back into shape. Horace, on the other hand, spent the past year training in Battleschool, traveling with Halt across Gallica and fighting the brigands there, then training with the Skandians before the battle (and given the timeline, it's possible he got to train with Thorn). And before that, he had advanced training from Gilan, an acknowledged Master Swordsman in Book 2! In light of all that, Horace's knighting is much less surprising. And his promotion to Royal Guard? Well, perhaps Duncan was a bit of a Shipper on Deck for Horace/Cassandra even then.
In Book 12, Halt brutally condemns Will for wanting to focus on finding Alyss' killer over his orders from the top, despite having done the very same thing in Book 3, and when the country was in much greater need of all hands on deck. On the other hand, Halt ensured that someone would take over his responsibilities instead of just ignoring the country's need entirely, and his mission was also to benefit Araulen by bringing back a highly-talented Ranger apprentice.
In Book 7, when Selethen forgets to take Cassandra's seal to the Wakir and she has to remind him, it's actually a subtle bit of foreshadowing: Since he is the Wakir, he doesn't have to take it anywhere.
Book 9 has Horace accuse Halt of being in Master Chubb's kitchen not to appraise Will as potential Ranger material, but actually just seeing him by accident as he planned to steal his pies. Given the debt he felt to Will's parents (as expounded upon in Book 11), it's entirely possible that Halt was there to keep an eye on Will (and also to steal some pies while he was at it).
In Book One, Halt tells Will "If you had lied [about stealing some cakes from the castle kitchens], you never would have become my apprentice." This despite Halt doing several worse things over the course of the series, such as having forged copies of several world leaders' personal seals, including that of his own king. What makes lying about stealing the cakes so bad? If Will had lied, he would have been doing so to avoid taking responsibility for his actions, making him no better than a petty thief. Halt, meanwhile, had performed his deeds to better serve his country. Deceit isn't necessarily a bad thing, but it can be if you're doing it for selfish reasons.
Book 8 mentions that Gilan has been the Ranger of Norgate Fief for the last couple of years. Whatever happened to Meralon? Well, it's entirely possible—if not probable—that after the whole incident with Keren, Crowley had Meralon reassigned to another, less strategically important fief and put one of his most trustworthy and competent Rangers there instead.
Stig's flying sidekick in "Duel at Araulen" seems like a case of Artistic License Martial Arts, as most trained martial artists seldom use flying kicksnote Once you're off the ground, you have little to no control over your own momentum. However, it makes sense when you realize that Skandians are trained in wrestling, but there's no mention of other hand-to-hand training. Even the Maktig contests are mostly wilderness survival and weapons fighting. Most likely, it's in-universe flynning.
Halt has a tendency to not handle sea travel that well, getting really seasick no matter what he tries. This is mostly played for laughs, until you realize that the last time Ferris tried to kill him, it was by forcibly drowning him in the river.