Scrolls is a game with more nuance and subtlety than meets the eye. Often, the misplays that set us up to lose are not the obvious ones, or even errors that often strike us as mistakes at all. Below are a few subtle tips on "what not to do"
Among the overlooked errors, this is without doubt the sneakiest and most detrimental to a game in progress. The "Poker Hand" occurs when you play out the entire contents of your hand at one specific moment- and then have to immediately topdeck the next turn, and sacrifice that Scroll for 2 Scrolls.
The mistake here is that you lose the ability to have significant choice in using the sacrifice mechanic (easily, the most powerful mechanic available to the player, given that it doesn't cost resources.) Often, this will put you behind in filtration and put you into difficult situations where you draw the most crucial scroll in your deck, but have to consider sacrificing it at the risk of getting even further behind on cards. It becomes extraordinarily difficult to consider sacrificing for resources, since you'll never really have much play in using those new resources.
To prevent this from happening- don't play everything in your hand, simply because you have the resources to play it- unless it wins the game on the spot, or (in some exceptions,) creates a massive advantage (like completely clearing a board.) Counting on effects like Hired Smuggler's pillage or Brain Lice can also provide some phantom cards to provide sacrificial power during your next turn.
Anyone who has played a significant number of games, have probably made this mistake more than they would like to admit. Often in a game against a particularly aggressive start, it becomes easy to lose resource awareness, because of a desperate dig for a hard counter to an early threat. This often leads to losses that involve having 8+ scrolls in hand, but 6 or fewer resources, and the inability to combine the tools available in any meaningful way.
To avoid this, simply take a more "big picture" look at the game every turn. Observe what your opponent is threatening to do, and mitigate it, (even if your best answer isn't on hand,) by simply playing your strategy well. Being aware of how many resources your strategy needs to work, and making sure to sacrifice extra scrolls for resources when you have plays, will ensure that your slower deck will have the opportunity to use all of the best tools, and bring the game to equilibrium. If not, nut-draws do happen- they're nothing to get upset over.
When positioning expensive units, a common mistake is to hide every unit behind cheaper units, even at the expense of losing overall board presence. Perhaps the most common example of this is the Decay creature "Cursemonger" - who totes a beefy 4 health.
Often the opponent will remove it whether or not it is tucked behind a cheaper unit- if they feel that it's important enough to remove, but being as large as the unit may be, often means that the opponent has to use several attacks, or cards to get it off of the field anyways- giving a net tempo and/or card loss if they do decide to stop you from having a larger reach on the board.
It's often important to remember that creatures naturally cover 2-3 lanes at a time, and covering as many lanes as possible reduces your opponent's ability to put objects out of your reach.
It is very important to choose for yourself whether or not the unit you try to protect, (by keeping in an area of the board you dominate,) really needs the protection in the first place. Often however, you have to weigh that against it's ability to net some good real-estate.
Cantrips (cards that have a "draw a card" effect attached to a regular effect) are powerful, often surgical utilities that allow you to increase the cardflow of your deck while doing something you already need to be doing. Many players often don't get real value out of their cantrips, instead trying to maximize cardflow- without paying attention to actual impact of the cards that they are playing.
The answer here is rather simple- don't play cantrips that you can't reliably get a valuable effect out of- and consider how often you just use them to continue digging for cards. If your sole use for them is to dig for more cards, then chances are you should just be playing more impactful scrolls in the first place.