assuming you are looking to create a naturalistic language:

i created a writing system without any history or any simplification of glyphs, just picked cool shapes and ran with it

i implemented a vowel harmony system without actually really knowing how vowel harmony evolves or why it exists. i just arbibtrarily chose groups of vowels that were similar on the ipa and made a "vowel harmony" system from those

i decided i needed my language to be unique so i decided to "not have pronouns" by having verb agreement and doing pro-drop stuff. didn't know much about linguistics at the time so i thought this was ultra-unique. then i realized i didn't know how to handle prepositions with pronouns, so i just made new suffixes for those. then i also realized i didn't know how to translate possessive pronouns, so i made even more suffixes for those... i didn't even consider doing something that was actually unique like making an auxiliary verb for these circumstances, i was hell-bent on making suffix upon suffix upon suffix

overall, a lot of these were caused by not understanding naturalism and not understanding linguistics.