Replacing blank textures fast?

Is there any quick way to replace unknown textures? I tried the "Find and Replace" but it makes a bunch of those one-sided wall things (like in MAP28 for example).

Those black exclamation marks on a red background denote missing textures. Are you replacing the textures with known ones , that are either in Doom2.wad or physically in your pwad ?

the main question was HOW to replace those textures fast, not what they are (except for the one fact that he put unknown instead of missing)

just highlight those lines in the editor and right click to open up the linedef editor. Go to the sidedef tab and replace the parts of the sidedef that say Missing Texture in red letters.

If the unknown textures have all the same name, and if they're really too many of them in various places you might want to consider the following. Add a dummy texture to your resource WAD with the same name, then use Find & Replace with the texture you intended to use. Remove the dummy texture from your resource wad afterwards.

Or you can select the sectors you have the error in and then enter the texture to use in the error fields. The result: All wrong textures are replaced at once.