Anki makes the (realistic) assumption that the intrinsic difficulty of the cards in a given deck are heterogeneous (or in other words, that some cards in your deck will be more or less intrinsically difficult than others), and accounts for this by using the grades you give your cards as feedback to constantly adjust the ease factor of individual cards (remember, ease factor accounts for the intrinsic difficulty of cards, and controls the speed at which intervals grow). When you grade a card "again" or "hard," the ease factor is decreased; when you grade a card "easy," the ease factor is increased. Only when you grade a card "good" does the ease factor remain unchanged. The idea is that, over time, each card will slowly move towards its hypothetical "ideal ease factor." It sounds nice in theory, but in reality, this system is highly flawed.
I explained above that because the equation for the forgetting curve only tells us the probability of how long we will be able to remember something, we have to decide how low are we going to let that probability drop before we review; by default, Anki aims to let it drop to 90%. Let's imagine that, hypothetically, we have an Anki deck in which every card has an identical intrinsic difficulty, and the ease factor of each card is perfectly matched to that intrinsic difficulty. Assuming that we are waiting for retrievability to drop to 90% before reviewing, on average, we would miss 10% of the cards that come up for review. In Anki, each time you hit "fail" on a card, the card's ease factor is reduced by 20 percentage points, making the card's interval grow slower (in turn making you review it more often), indefinitely from that point onward. Hopefully you see the problem here. This means that, in this hypothetical scenario where every card in the deck already has the perfect ease factor, Anki is outright messing up the ease factor of 10% of the cards you review each day, forcing you to waste time reviewing them more often than necessary in the future.
The SRS isn't about not forgetting, it's about forgetting strategically; because the price of retention is time, aiming for 99% retention is inefficient and unrealistic. Thus, Anki is calibrated so that you strategically forget 10% of your cards. Although some of that forgetting will be due to inappropriately high ease factors, we can expect a large portion of it to simply be a function of the forgetting curve, completely unrelated to ease factor. This is why the more recent SuperMemo algorithms take the entire history of the card into account when deciding whether or not to update the A-Factor (SuperMemo equivalent of ease factor); if a card was graded "good" 10 times, and then "again" once, chances are that the ease factor doesn't need to be changed. On the other hand, Anki lowers the ease factor of every card that is failed, regardless of the card's history.
Another way to think about what is going here is to borrow the concept of "learning rate" from machine learning. The following is a quote from a Slate Star Codex article:
"Suppose that you have a neural net trying to classify cats vs. dogs. It's already pretty well-trained, but it still makes some mistakes. Maybe it's never seen a Chihuahua before and doesn't know dogs can get that small, so it thinks "cat". A good neural network will learn from that mistake, but the amount it learns will depend on a parameter called learning rate: If learning rate is 0, it will learn nothing. The weights won't change, and the next time it sees a Chihuahua it will make the exact same mistake. If learning rate is very high, it will overfit. It will change everything to maximize the likelihood of getting that one picture of a Chihuahua right the next time, even if this requires erasing everything it has learned before, or dropping all "common sense" notions of dog and cat. It is now a "that one picture of a Chihuahua vs. everything else" classifier. If learning rate is a little on the low side, the model will be very slow to learn, though it will eventually converge on a good understanding of its topic. If learning rate is a little on the high side, the model will learn very quickly, but "jump around" between different understandings heavily weighted toward what best fits the last case it has worked on."
Going with this metaphor, we could say that the learning rate of SuperMemo's algorithm with regards to ease factor is relatively low. A disproportionate amount of weight is not placed on the most recent data point; all data that has been collected up to date is taken into account when deciding whether or not to alter the ease factor, allowing the ease factor of cards to be efficiently optimized in the long run. On the other hand, we could say that the learning rate of Anki's algorithm is extremely high. Each time you give a card a grade other than "good," Anki makes a short-sighted shift to the ease factor in order to honor that most recent grade, ignoring all of the other data that has been gathered up to that point. By ignoring the possibility of the grade being accounted for by proper functioning of the forgetting curve, and always aligning the ease factor to the single newest piece of data, the algorithm increases the possibility of short-term success with the card, in turn majorly sacrificing efficiency in the long run.
This is what I call the "ease factor problem": most of the time, when you forget a card, it is either due to poor initial learning, a random brain fart, or a simple fluke of memory, not because the card is inherently harder than the other cards in your deck. As ja-dark put it, "When you fail cards, it's commonly a matter of the quality of your memory encoding; that the interval was too long was a symptom of this, not the cause." So what you really want is to see the card again once, properly relearn it, and then have the card's interval grow at a normal rate. Because the lapse was a fluke, the measure taken should be temporary, not permanent. But in reality, that fluke is going to cost you 20 percentage points off your ease factor, causing you to waste time needlessly seeing the card more often for the rest of time.
In the worst cases, the ease factor problem can lead to what Conaanaa refers to as "ease hell," where due to successively grading a card "again" and "hard," the ease factor is reduced to the point where you are stuck seeing the card every few days, indefinitely. The following image, taken from Conaanaa's video on the topic, demonstrates the radical difference in interval growth between a card with a healthy ease factor of 250%, and a card with the minimum ease factor of 130%. Starting with an interval of five days, after four repetitions, a healthy card's interval will have grown to 75 days, whereas an unhealthy card's interval will have only grown to 10.