Alrighty then, I've had some time to mull this over and am now in front of a PC instead of a phone.
Before I give my two (or seven) cents, I want to be clear:
I do not endorse cheating, deceiving ("You asked if I had any teleporting units, but not if I had any hidden glades movement units- ha!"), or playing psychological games. Nor do I believe in treating a new player who is asking you to help them learn the same as an opponent in a tournament.
I'm not about being "that guy."
It's probably good to keep that in mind when/if you read this to the end. In no particular order, here are my thoughts on gotchya moments and avoiding them (or not) -
I believe that using deceit to create an intentional trick situation is awful. I also think that there is more than just a little onus on the players to follow the rules, to know the rules, and to gain experience with them.
In the olden days, list sharing was not only not required, but not particularly encouraged. Many tournaments would require you to swap lists after the game as a way to discourage cheating. Sharing was not in the rules and was only an expectation when playing a small subset of the ways the game could be played. Today, this is not the case. The actual rules of the game now tell us to share our lists before we play. Wow! That was a big change for old-timers like me. A welcome one in the end, but a big one. So, we have a base level expectation that you will share your list with your opponent. What that opponent does with the knowledge you give them in your list is up to them, not you. You could even take the extra step of giving them an AoS Reminders sheet, but that is up to you being generous. I have no problem doing that.
I will also answer questions my opponent has about the rules and stats of a unit, but almost never (exception as listed in the intro) what tactics I plan to use. What I won't do is run down the highlights of my army proactively. There are two main reasons why.
1. Princess Bride. Sort of. If I offer up proactively what the highlights of my army are, I can quite easily, intentionally or not, tip my hand as to what I plan to do. I am naturally going to want to talk about the coolest things my army can do, which are likely the things I like about the army and will try to do. I'm often giving my game away before we start if I do this.
On the other hand, if I am a ****** (and I've seen this all too often, sadly), I can give my opponent a red herring of sorts by highlighting the things I want them to focus on while I am secretly planning to do something else. I tell them that Unit X can teleport behind their lines, so they guard against it, committing troops to defending the rear, while all the while I have zero desire to do that and now I get to worry less about some of their forces. Shady as heck.
It's best to not proactively share capabilities in order to avoid being on either side of the psychological rope bridge.
2. Experience matters. This is not "git gud." This is recognition that we grow more from experience than being told what to do. We also gain knowledge of what other armies can do by actually playing against them and, yes, getting burned by something you didn't see coming. It's not fair to the experienced/informed player to give up that advantage that they have earned through playing, reading, etc. It's also not fair to the less experienced/informed player to deprive them of a chance to improve through actual game play. Again, accumulating knowledge through actual experience (and research) is a part of growing as a player, and it should be both rewarded and earned.
Next, secret info. Another old time thing was that information used to be quite deliberately kept secret. If you had a magic item that could burn out after using it on, say, a roll of 1, you were under no obligation to let your opponent know if it had failed. The Staff of Volans is a great example of this. When you used it, your spell could not be dispelled. That meant your opponent would not waste dispelling resources stopping a spell cast with it until the staff had burned out. You had to decide to commit the resources without knowledge of the staff's status. If it had not burned out, you wasted your resources if you used them. If it had and you assumed it was still functional, you might not use resources and the spell would be cast unchallenged even though the staff could not power up the spell. It was a guessing game based on not sharing info. Heck, in those days we didn't even use dice to cast and dispel. You had cards that were dealt secretly that governed the winds of magic. My point is, the game used to be designed with hiding info as part of the experience, and for many veteran gamers this idea of freely sharing info is tough to get used to. I'm not saying that justifies bad behavior, only that in the OP's case, the opponent may have had some residual influences knocking around. Maybe not, but maybe.
I also want to draw a distinction between hard rules and optional choices. Another poster above talked about reminding the opponent he has Nurgle saves to make when the opponent forgets to take them. That's not just courtesy or an option. That's the rule. If you know they get a save that they forgot to take, you are obligated by the rules to inform them. This is not a case of the opponent not learning new tactics, new options, having a brain glitch, making an error, etc. This is them unknowingly skipping a rule, and both players have a responsibility to enforce the rules. If you are inwardly gleeful that your opponent forgot to save and you win as a result, you have cheated because you knew the rule for resolving that save and chose to ignore it.
That's it for now.
Don't cheat. Don't deceive. But also don't grant unearned advantages or play psychological games.
Winning is the object of the game. Having fun is the point of it. Cheating or defeating yourself is against both.