Tools
Why it appears: You can't use most tools on a Type layer.
How to fix it: The dialog is suggesting that you rasterize the Type layer (Layer>Rasterize>Type), but I recommend that you duplicate the Type layer by pressing Command-J (PC: Control-J) before rasterizing, just so you have a backup plan.
How to prevent it from happening: It's best to remember that few tools work on a Type layer—basically only the Type tool (T) works so you'll get the same warning for all tools.
Why it appears: In Photoshop CS2, you can have more than one layer selected, but when you do, the majority of tools will not work.
How to fix it: Make sure that only the layer you want to work on is selected in the Layers palette.
How to prevent it from happening: Take a look in the Layers palette to ensure only one layer is selected before using any tool.
Why it appears: If you selected text in a Type layer with the Type tool (T) and then applied the Faux Bold (or Faux Italic) style (by choosing it from the Character palette's flyout menu), effects such as Text Warp will not work.
How to fix it: This is one of the few warning dialogs that actually offers to solve the problem for you, by removing the Faux Bold style.
Why it appears: You try to use the Move tool (V) to move the Background layer (or any layer that is locked, meaning the Lock icon appears to the right of the layer's name in the Layers palette).
How to fix it: If you're on the Background layer, either duplicate the Background layer (Command-J [PC: Control-J]) and work on that copy, or unlock the Background. (One quick way to unlock the Background is to double-click on its name and rename it in the resulting dialog, which makes it an editable layer.)
Why it appears: You already have the maximum of four color samplers and are trying to add another one with the Color Sampler tool (Shift-I).
How to fix it: Delete an unnecessary color sampler by Option-clicking (PC: Alt-clicking) on a sampler with the Color Sampler tool. Or, with the tool active, you can click on the Clear button in the Options Bar to get rid of all the samplers.
How to prevent it from happening: Make sure you only have four samplers before you click again!
Why it appears: You must press-and-hold Option (PC: Alt) and click in an area on your document to establish a source for the Clone Stamp tool (S).
How to fix it: Make sure you're holding down Option (PC: Alt) when you click with the Clone Stamp tool to select a source.
Why it appears: You must press-and-hold Option (PC: Alt) and click to establish a source for the Healing Brush tool (Shift-J).
How to fix it: Make sure you're Option-clicking (PC: Alt-clicking) to define a source before you begin painting with the Healing Brush.
Why it appears: The layer you're trying to work on is locked.
How to fix it: With the layer selected, turn off the Lock icon at the top of the Layers palette to unlock the layer. Now try again.
Why it appears: You're trying to use a tool with a Group active in the Layers palette rather than a layer.
How to fix it: You must either expand the Group to be able to click on a specific layer (just click on the right-facing arrow to the left of the Group's name) or use the Merge Group command (found under the Layer menu), remembering that the layers in the Group will turn into one layer.
Why it appears: You have more than one layer selected in the Layers palette.
How to fix it: Make sure that only one layer is selected in the Layers palette. Note: Some warning dialogs will name the specific tool.
Why it appears: You're trying to use the History Brush (Y) after cropping the document to a different size or adjusting the canvas size (Image>Canvas Size).
How to fix it: You have to choose a different History State as your source in the History palette that appears after you altered the document's size, or you will not be able to use the History Brush.
Why it appears: You're trying to use the History Brush (Y) after changing the color mode of the document.
How to fix it: You have to choose another History State as your source in the History palette—one that appears after you changed the color mode—or you will not be able to use the History Brush.
Why it appears: You're trying to use the History Brush (Y) after adding or deleting a layer in the Layers palette.
How to fix it: You have to choose another History State as your source in the History palette—one that appears after you added or deleted the layer—or you will not be able to use the History Brush.
Why it appears: When using the Make Work Path command (from the Paths palette's flyout menu) while you're using a drawing or selection tool, you're asked to enter a value for Tolerance. If you choose a value that's too low or too high, you'll get this warning.
How to fix it: Enter a value from 0.5–10 pixels in the Tolerance field of the Make Work Path dialog.