> For the complete documentation index, see [llms.txt](https://help.lob.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.lob.com/print-and-mail/designing-mail-creatives/creative-formatting/creative-errors.md). # Creative errors Lob occasionally encounters problems with the creative of a mail piece not meeting print specifications; this can occur with either PDFs or HTML. To avoid sending out erroneous mailpieces, Lob provides visibility into when and why these mail pieces failed. To learn about all the ways you can gain visibility into your failed orders, view the Failed Orders documentation. {% hint style="success" %} We always recommend testing; and where possible, sending yourself a printed mailpiece. {% endhint %} ## **PDF** **Lob will fail incorrectly formatted PDFs.** [See our PDF preflight checklist here](https://help.lob.com/print-and-mail/designing-mail-creatives/creative-formatting/pdf-preflight-checklist) to ensure your file will process through our rendering engine. ## **HTML** **Lob will fail HTML that includes inaccessible assets (images, fonts, any other URLs):** * the link to an asset is incorrect * the asset referenced in the file is missing * Lob has not been given the correct permission to access the asset (ensure that the link is set to public viewing, and not private) * when a remote asset references a local asset (for example, a remote asset is at [www.example.com](http://www.example.com) which then references something local in that directory) * slow or under-resourced server providing the asset (we recommend using a performant service, such as Amazon S3, to host the content) ## **HTML Template Proof** If your HTML contains an inaccessible asset, your HTML proof will show an error with an error reason. For details on HTML template proofing, look [here](https://help.lob.com/print-and-mail/designing-mail-creatives/maximizing-engagement/dynamic-personalization#template-proof). ## **In the Campaigns UI flow** * **When creating your Campaign, you *****may***** be notified when you generate a Creative Proof** (if the sample mailpiece that is rendered results in an error); if this occurs you should troubleshoot against the above list.