I've never had to use "hyper triggers" before, this isn't my first time making these kinds of maps mind you, however this is the first time I've had this problem, regardless I gave up and just used an "elapsed" time thing anyway
Well, you do need to use hyper triggers when checking for an exact Countdown Timer value. The exact same thing will happen with Elapsed, so if you continue using it without hyper triggers, you'll eventually run into the same problem.
Hyper triggers make StarCraft check the trigger conditions 12 times per second. Without hyper triggers, it checks much less frequently. When you check for "countdown timer is exactly 10 seconds", it will only be true for 1 in-game second (in-game time runs faster than real time), when the timer is between 11 and 9. However, without hyper triggers, it only checks the conditions every 2 in-game seconds.
Hopefully you see the problem here. Any "exactly" value for the countdown timer will only be triggered half the time. The times it is triggered, it will always be triggered (assuming the countdown timer always starts at the same point, like the beginning of the map). Half the values you could use will not work. The same issue is true for Elapsed Time, which is basically just an invisible countdown timer that counts up from the map start, using in-game time.
It's fairly easy to create a map where you have a bunch of triggers based on Countdown Timer, checking it every 2 seconds on average, which will all never fire. They generally won't be exactly every other value (11, 9, 7, etc), due to an offset between the cycles and the timer, but it will still end up being half the values which fail.
In fact, it's easy enough that I'll just go do it now.
(Several minutes later) Done. I've attached the map below. Open it and look at the triggers. They all use "countdown timer is exactly X seconds". Be aware that no Marines will ever be created.
Hyper triggers should be used in every map (and these days, they are). There's really no excuse to not be using them (there technically can be but you really don't need to worry about it at this level of mapping). It'll improve your map's quality and the players' experience.
If you have a proper amount of sense, then you're certainly going "Oh crap, this sucks! How do I implement hyper triggers to resolve these sorts of problems? How do I make my maps more awesome like this? Is it hard?!
"
This is where you luck out. Hyper triggers are incredibly easy to add to your map. It'll take you approximately 30 seconds. First, create a new trigger for All Players, with the condition Always, and the actions "Wait 0 milliseconds" 63 times and "Preserve Trigger" 1 time (you can also replace one of the "Wait" actions with a "Comment", for the sake of making it look smaller on the trigger list). Second, copy this trigger 3 times (for a total of 4 triggers). You now have hyper triggers.
Something important to note:
If you have a player who is always in the game (like a computer player), and they have no Wait conditions in any of their triggers (other than the hyper triggers), then you should use that player instead of All Players. This will make your life a lot easier.
Ideally, you should be using death counters instead of waits anyways, as jjf mentioned earlier.
However, if you are already using waits and don't want to switch, and all the players have waits, then just leave the hyper triggers as All Players. The caveat to this is that the hyper triggers must be moved to the bottom of the trigger list (they will be at the bottom by default, but you'd need to move them back to the bottom every time you made a new trigger which included waits in it).
At any rate, you should start using them. Your ability to map is incredibly limited without hyper triggers.
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