Other Issues.
• Most Preview actions compress a JPEG once. But if it's already been edited in iPhoto, it will not be compressed again.

• Some of the Apply ColorSync Profile actions (Preview) don't work in succession. You can follow a Lightness Increase action with a Black & White action. But you can't following a Black & White action with a Blue Tone action.
Action Quirk Glossary.

If your workflow should logically work but doesn't, check here for issues with actions that may be causing the problem.

Ask for Photos (iPhoto)
This doesn't register the last round of changes in iPhoto's library until iPhoto is closed. You can close iPhoto while the workflow is running and the changes will show.

Combine Mail Messages (Mail)
Requires a "Display Mail Message" action before it to work.

Copy Finder Items (Finder)
It only copies items to a new location. It will not copy items to the same folder.

Create Archive (Finder)
It doesn't want you to put the archive where the original files you're putting in it are located. If you do, the action will check as if it worked, but will fail to produce a zipped file.

Download Pictures (Image Capture)
Always loads images into the Pictures folder. Follow it with a Move Finder Items action if you want your images elsewhere.

Get Contents of TextEdit Document (TextEdit)
Get raw text of an open TextEdit document. It does not retain images or text styling.

Get Selected Finder Items (Finder)
This is usually not needed in the middle of a workflow to pass items between actions. Here it usually screws things up. It's best used at the start of a workflow to be saved as an application to call up highlighted items to be processed. The same applies to any of the "Get Selected..." actions.

Group Mailer (Mail)
Only works with the New Mail Message action before it. There currently is not an action to send an existing mail item to one or more recipients. Group mailer duplicates the new message and sends an individual copy to each recipient, unlike send to a group in the Mail app, which multiple addresses the same message.

Mount Disk Image (Finder)
Anything after this in a workflow tends not to work. If you're having and issue, either call up the second half of your workflow in another workflow or use the Open Finder Items action to launch the disk.

New Mail Message (Mail) Any files in the workflow passed to the New Mail Message action will be attached to the new message. If you don't want the previous files attached, choose Ignore Results by clicking the Files/Folder button on the top right of the action.

New PDF Contact Sheet (PDF) This creates a high quality PDF page of Images that can pretty much be printed poster size. But the file is huge. It can easily exceed 100 megs. If you add a Compress Images in a PDF Document action after this in the workflow, the file size can be reduced by as much as 90%.

Render PDF Pages as Images (PDF)
Some actions that create new PDFs don't appear to work. But these actions create PDFs and put them in a temporary folder. Automator keeps a reference to them, so this action should be followed by an action that uses the file or moves them to a place you can find them. Use Move Finder Items action to put the PDFs where you can find them. You can also follow this with a Rename Finder Items action, since the PDFs will have goofy names that have nothing to do with the original name. Or you can use Add Attachments to Front Message to put it in an email. Or use Open Finder Items to view it.

Rotate Images (Preview)
Doesn't seem to work with single-page PDFs. (thx: Fabrice Nordmann) Compresses JPEGs it processes.

Scale Images (Preview)
Has a bad habit of resetting the scale value, especially when Percentage is selected. Check and reset before you re-save a workflow. If you set scale to 100%, JPEGs are compressed and all images passed will most likely inherit a thumbnail.

Take Screenshot (System)
If you save the screenshot as a file, it's a PNG. If you save it to the clipboard, it pastes as a TIFF into a Cocoa app, such as TextEdit. In the Interactive mode, this also has a bad habit of resetting the area of the capture from Selection to Window. You may have to set it back each time you save the workflow or app.

Update iPod (iTunes)
Due to a different sync method, this will not update the iPod Shuffle when plugged in and already updated.
However, Add Songs to Playlist action will update the Shuffle.

Watermark PDF (PDF)
Some actions that create new PDFs don't appear to work. But these actions create PDFs and put them in a temporary folder. Automator keeps a reference to them, so this action should be followed by an action that uses the file or moves them to a place you can find them. Use Move Finder Items action to put the PDFs where you can find them. You can also follow this with a Rename Finder Items action, since the PDFs will have goofy names that have nothing to do with the original name. Or you can use Add Attachments to Front Message to put it in an email. Or use Open Finder Items to view it.