White lines across the design have appeared in my canvas, not a pattern.

Hello @10792232,

We understand that encountering technical issues can be frustrating. Would you mind trying to press the Cmd+E keys and checking if it helps?

If yes, kindly try manually resetting Illustrator's preferences and share your observations:

• Close all Adobe applications.

• Go to the following locations

• ~/Library/Caches

• ~/Library/Application Support/Adobe

• ~/Library/Preferences

• Rename Adobe Illustrator, com.adobe.illustrator and Adobe Illustrator 27 Settings folders to Adobe Illustrator.old, com.adobe.illustrator.old and Adobe Illustrator 27 Settings.old

• Launch Illustrator from the Creative Cloud.

Note: Location starting with this "~" sign indicates the User Library, which is hidden. So, you need to copy & paste the exact path in spotlight search or from Finder Menu > Go > Go to Folder > Paste the location in the dialog box like this:

Disclaimer: Please note that renaming preferences folders will remove all the custom settings, and Illustrator will launch with default settings. You can also save a backup of the folders in case you want to. The location is mentioned above.

Looking forward to hearing from you.

Thanks,

Anubhav

Is that a large image?

What's in the file?

What kind of file is it?

Did you flatten transparency?

Is this a pattern?

Without a lot more details this cannot be solved. And this snippet of a screenshot is just not helpful.

Can you select the line? Is it visible in outline view? In the Layers panel?

Does it go away when you Preview on CPU?