If mistakes means picking parts that are not necessary or for which better alternatives exist.
These planning mistakes are something that I observe all the time in forums like /r/buildapc. Avoid them, and you will end up with a faster system that probably even costs less. Most of this also applies to build that don’t target gaming first.
1. Ignoring the Ryzen 5 3600
6 cores, 12 threads - for a gaming system that is an excellent combination. Right now there are no games that need more cores, if the limit gets raised the 12 threads provide additional buffer. Think about how many systems are out there that have only 4 cores and threads, or just 8 threads. Even the rather new i7-9700K has only 8 cores and threads. In practice that will most likely give the Ryzen 5 a very long life, and its performance right now is very similar to what the more expensive options would provide.
2. Picking the Ryzen 7 3800X or Ryzen 5 3600X
Sadly, some options in AMDs current lineup are traps. Parts that should never be bought, because the alternatives are just better deals, at least as long as there are no special deals that change the equation. The first and most common of these is the Ryzen 7 3800X. It’s 40 bucks more expensive than the Ryzen 7 3700X, but not significantly faster. That already follows from the specs: a measly 100 MHz turbo clock difference. And then there is the automatic overclocking on Ryzen, further diminishing the bigger processors lead. In the end you end up paying for just the higher number, as Steve from Gamer Nexus put it.
The 3600X is basically the same story: More expensive for a minimal advantage. Mostly the better cooler, but for the usual price difference you could get a better aftermarket cooler.
You can double check that advice by look at the FPS numbers in their benchmark comparison.
3. Not getting a Ryzen processor
This happens less often now, but it still happens: Most of the time you want a Ryzen processor. AMDs current generation, Ryzen 3000, has cores as strong as Intel, but the prices are most of the time better. When there is a performance difference, like with the Intel Core i9-9900K, that’s because of the processor being pushed to the limit - which results in high power usage and heat output. The resulting FPS increase is irrelevant for most gamers.
There is one processor that looks like an exception: The i5-9400F. That is one processor with an attractive price and performance. But: Its stock cooler is bad, its price is less attractive if you factor in that you need a cooler as well. And it has only 6 cores and threads, while with the slightly faster Ryzen 5 3600 you could get 12 threads, which is good for future proofness. So in the end again you arrive at the AMD offering being more attractive.
4. Too much memory
Current games do in general not need more than 16GB ram. There are some exceptions, like a heavily modded Cities: Skylines. But for most, 16GB is not only enough, it’s plenty. If you buy more - that’s usually a 32 GB kit then - it’s just wasted money. Because if you follow the recommendation to get a board with 4 ramslots, upgrading the memory when it’s needed is the easiest PC upgrade possible. It’s something that can easily wait.
5. Too slow memory
Some locked boards support only slow ram, B365 for example is limited to DDR4-2666, faster ram will get clocked down. But the usual choices like B450 and X570 for AMD, Z390 for Intel support faster ram easily. And it can have a very big effect, especially for Ryzen processors, that get a nice FPS push from being combined with faster ram.
That also includes getting just one ramstick. It should be two, because with two sticks you get dual channel, which further improves performance. 4 would also work, but then you limit upgradeability too much.
6. Too expensive memory
On the other hand: If you can get DDR4-3200 (ideally with a CL 16 latency or better) for a reasonable price there is no big advantage in getting even faster DDR4-3600 kits. Those are usually much more expensive and their performance impact is very limited. To go even faster? Can have even negative effects, and now it gets really expensive. No, the current optimum is a cheap DDR4-3200 16GB kit.
There is also no reason to get a more expensive kit that has the same specs. Most of the time, it’s the same stick with a different branding.
7. A bad power supply
Don’t save on the power supply: A bad power supply will seldom explode (but it happens!), the more common effect of cheaping out here is the other parts dying prematurely. There can also be other “fun” effects like coil whine. What I see especially often is planners getting lost here and picking old recommendations. The one I hate the most is the Seasonic S12II, M12II or Seasonic Evo. All of these are very old. They were okay options when they were released, for budget builds. In 2020 they are not suited even for those. They still get picked based on old reviews and because Seasonic usually makes excellent units, despite being outdated and overpriced.
Good power supplies are modern units with positive reviews, like the Corsair RM 650 or the cheaper and still very good BitFenix Formula 550. With the current supply crisis making good units unavailable one can also look for reasonable alternatives, like Seasonics Core GC/GM series, that miss proper reviews so far but are reported to be based on the good Focus architecture.
8. An overdimensioned power supply
Almost never a psu too small is picked. What happens instead is that planners splurge on the power supply: Not necessarily on quality (see the point above), but on wattage. And then a system that will maybe use 350W under full load gets a 1000W platinum unit for 300 bucks. That’s not necessary. 650W is plenty for a single gpu system (and you never want dual gpus in a gaming system, that doesn’t work properly), 550W almost always enough, and more often than not 450W would also be completely fine. And Gold efficiency is a good target, more costs more than the higher efficiency can make up for over the lifetime of the power unit.
9. Sticking to SSD + HDD
For a long time the rule was: Add an SSD to your build, you will get a more responsive system and you can store your favorite game there, to reduce loading times. That advice is outdated! Now, SSDs became so cheap that they should be used exclusively. All your games deserve to be stored on an SSD, and most modern games will have very long loading times if they are run from a slow HDD. So drop the HDD and use the money to get a bigger SSD instead. It will also result in a quieter system.
10. Getting a high end SSD
On the other hand, gamers that heard about the advantage of SSDs tend to pick a very fast SSD, often the popular Samsung 970 Evo. Small sidenote: There are two protocols that a SSD might speak, SATA or NVMe. SATA was first, NVMe is faster. The 970 Evo is a fast NVMe drive. But it turns out: In practice, the difference between a fast NVMe SSD, a slow NVMe SSD and even a regular SATA SSD is unnoticeable. You can measure it, sure, but you can’t feel it when using the system, and that includes gaming. So: Since SATA SSDs are not cheaper anymore there is no reason not to get a NVMe drive, but it can be a slower model like the Crucial P1.