Common mistakes:

• using PlayerPrefs for huge save data;
• saving inside Assets;
• saving every frame;
• not checking File.Exists;
• storing GameObject references;
• no versioning;
• no data validation;
• confusing ScriptableObjects with save files;
• ignoring file errors.

Prefer:

persistentDataPath
JSON/files
stable IDs
versioning
validation
reasonable autosave