Sometimes we start building before we really think things out, and we end up giving ourselves more work because of it. What are some examples of areas in your project that you caused yourself delays by not taking the time to plan ahead?
Some big things that's come back to bite me in my tail later:
Tons of old projects: not using Git or some other source control. I made a big change, broke something, gave up, and had no way to get rid of it without doing a lot of extra work.
YaruSMB & Shattered World: using globalvar everywhere. Made it basically impossible to get the project working in Studio later.
Daemon Detective Gaiden: using GM's built-in collision checking. The original version was under a tight jam deadline, so I cut a lot of corners to get the game done quickly. But the built-in collision system had issues with a lot of things like moving and jump-through platforms because I didn't have full control over when collisions or movement happened, forcing me to spend a lot of time adding more and more workarounds on top of the original collision system.
Gun Princess 2: deciding to make the game party-based instead of single-character halfway through development, forcing me to turn everything into arrays, and then afterwards realizing it didn't really add anything to the core gameplay loop.
Sonic 1 3D Remake: saving resource IDs directly into level-editor files. After GMS1 started making resource IDs change each time you compile, the entire file format was broken and the level editor was useless without major rewrites.
Gun Princess 1: doing the entire game in drag-and-drop. Later on in the project I needed to refactor some huge D&D sections, like the logic to compute the ammo cap depending on currently equipped weapon, and it's really hard to move around big chunks of D&D actions - text you can just cut-paste.
Final Columbus: not knowing when to quit. I kept adding more and more secrets to the game and it's still not quite finished, but a lot of the secrets are buggy to the point you can't even access them because other things changed. The excrutiatingly slow-paced intro makes a lot of people quit before they can even start finding all the secrets I added later. Also, the game was originally made for a jam with a "you can only use one button" theme, which forced some strange design decisions onto the menus and makes you have to pause the game way too much. Being a jam game, some things were hardcoded to speed things up, which makes the logic for which weapon you're selecting in the menu to be pretty nightmarish after I added alternate characters with different weapons and weapon upgrades.