File has "holes"
Text files are supposed to contain only characters, punctuation, numbers, etc. without any unrecognised ones such as character(0). The problem could have arisen because it was transferred from one system to another, part of the disk is corrupted, or else maybe the file contains unrecognised graphics (or else it is not a plain text file but e.g. a Word document).
You can solve this problem by converting the text using the Text Converter. If it is a plain text with holes these will be replaced by spaces. You can find texts with holes using the File Utilities.