If you've ever received a Photoshop Action from someone else, bought a custom Action,or downloaded one from the 'net, you've probably noticed that sometimes they don't work quite the way they were described. Usually, that's not because the Action is "wrong"; it's typically due to some assumptions being made when the Action was recorded.
Unfortunately, figuring out exactly what the problem is can be tricky, as most of the typical symptoms come from very different causes. Because of that, most of the problems described here identify the root cause rather than symptom.
The Action assumes the image is a single layer.
Many Photoshop still users do most of their work on a single layer. If the Action assumes the image is in just one layer, it may be set to only operate on the pixels in that layer. In those situations, it may not operate correctly if
These approaches are usually much easier than tweaking multiple different steps in the Action to work around the problems.
The Action assumes a particular color mode.
Many Actions only work in a specific color mode. (Usually--but not always--RGB, rather than grayscale, LAB, CMYK, or Indexed Color.)
The Action assumes a particular bit-depth
Many Actions only work properly in 8-bit RGB mode. Photoshop CS added support for 16-bit Layers, which avoids some of the problems, and CS2,CS3 and CS4 updated many more filters to work with 16-bit or 32-bit files, but not all filters work in those modes.
Set the file to 8-bit RGB mode before running the Action
The Action assumes a particular resolution setting and/or output size
Most Actions containing sharpening, feathering, blurring, and the like assume a particular "DPI" or resolution setting. An image whose resolution is significantly different than originally intended (a 600 pixel web-sized image, for example, when a 8x10 at 300 DPI print is intended) will often give very strange results. Selection edges won't be soft enough (or will be so soft as to be useless), type won't behave as expected, sharpening will be much too strong or too weak, etc.
Ask the author what the expected size and/or resolution is, or do a lot of trial-and-error to find the expected ones.
The Action assumes the composite "channel" is active
If you're working on a channel other than the composite RGB or CMYK "channel"--whether you're on a masking channel or a specific color channel, many Actions will not operate as expected.
Select the composite channel and deselect the others
The Action assumes a selection has been made, or that no selection is active.
Many Actions expect a selection to have been made before running them. Others expect no selection has been made.
If you have a selection, try clearing the selection before running the Action.
If you don't have a selection, try making one before running the Action.
The Action assumes the default colors (Black foreground, White background)
Many filters use the default colors for their operation. If the colors are set to anything other than the default, odd results may occur. (Actions containing Clouds or Diffuse Glow are common places for this to show up.)
Set the colors to the default. Inserting a new recorded step to set the default is even better.
The Resolution setting of the image is different than what was recorded
If an Action is recorded and uses a font at a specified point size (rather than pixel size), if the file it's being run on has a different resolution setting, the text may appear jumbled or overlap itself.
Set the file's resolution before adding the text. (The trick is finding out what the proper resolution was, as text in a 300 "DPI" files sometimes fails badly in a 301 "DPI" file.) Record a new step to set that correct resolution before the text is created.
The Action assumes a particular font is available on your system.
When you run an Action which uses a font which is not loaded, the results can be surprising. Even having a font with the "same name" (Times New Roman, for example), but in a different format (e.g. TrueType, OpenType, or Type One) can cause unexpected results.
Using "Record Again" on the specific step in the Action that specified the font, then specifying a typeface you do have loaded will usually fix this problem. (Double-clicking on the type step will also permit this.)
The Action was created before CS2 and is being run in CS2 or later
CS2's type handling changed from earlier versions of Photoshop. Sometimes, this results in Actions containing type not working properly: the text appears jumbled up, and all stacked together. This problem remains in CS3 and CS4 for those older Actions.
So far, the only workaround I've found is to record a NEW type step in the action; re-recording the existing one via Record Again does not reliably fix this problem.