A while ago, I made a guide with 
common platformer mistakes made frequently by people, as well as how to fix those problems. Unfortunately my editing time ran out before I could add more, and BH hasn’t responded to my wiki request.
So:
Here are 8 more common mistakes that people make in their maps, as well as how to fix and improve them
1. Permanent speed modification
Problem:
The player’s speed is permanently set to something higher or lower than the original speed.
Solution:
Don’t permanently set the player’s speed to something different. Most people are used to playing with a regular speed, so changing it can affect their playing ability. Even a minor adjustment is very annoying.
2. No collision props
Problem:
Some props have no collision, causing the player to “fall through”
An explanation is needed for this one
Explanation:
The reason why this is problematic is because people will make it too subtle or not subtle enough.
Too subtle, and players will fall through every single false prop, which is very annoying because you can’t tell which props are the offenders.
Not subtle enough, and players will just jump over every false prop/barrier, making the entire point of them useless and wasting memory.
Solution:
Just don’t use them. [1] They’re just annoying in general.
3. NO LAYERED TERRAIN
Problem:
terrain no layered. Too lazy lol.
Solution:
I’m not even gonna Sugarcoat this
LAYER YOUR TERRAIN!
DON’T BE LAZY
FOR THE LAST TIME. PUT SOME COMMITMENT IN AND LAYER YOUR TERRAIN
IT’S NOT THAT HARD
4. Too much directional consistency
Problem:
When ascending [2] the map, you only go this way 
until the world border stops you, and then you go this way 
Solution:
Give your map more direction than just one. Try not to make it repetitive.
Here is an excerpt from top z guide on platformers that illustrates this point
5. Longgggg falls
Problem:
The only place that respawns a player is at the floor, making you fall longer the higher you go.
(DLD actually has this problem, which is funny.)
Solution:
Place an overlay that says “respawn” that’s wired to a respawn device.
Even better solution:
Put your respawn zones [3] way closer to the action, instead of on the floor.
6. Excessive backtracking
problem:
After going through a large section of the map, players must go back to continue.
Solution:
Try not to have too much backtracking. If any at all. It’s boring to go through the same stuff twice, and It can be very rage inducing if the section is difficult.
Special thanks to @ClicClac for giving this suggestion!
7. Sentries falling through
Problem:
Due to a glitch, sentries will fall through terrain if not placed correctly, leading to problems.
Solution:
Instead of placing the sentry directly on the platform, place it slightly above like so:
8.Checkpoints that stay on
Problem:
Once you get to a checkpoint, it doesn’t deactivate.
Solution:
Have the checkpoint deactivate after a player touches it. It’s extremely annoying to reach a farther checkpoint, fall down, accidentally hit a previous checkpoint, and have to go all the way back up to the next checkpoint.
Literal rage quit.
Want more mistakes fixed? check out my previous guide below, as well as these other great guides on platforming
and sometimes directional consistency is good on those maps, as long as its a short one. But i usually agree.
