Apple's TextEdit program is a basic text editor included with OS X, which like WordPad for Windows can be used to quickly create text documents in plain text (.txt) or in rich text (RTF) formats. Similar to Preview for image files, TextEdit is commonly used as a starting point for opening various supported file types in order to preview them and make small edits to their contents. This helps when you need to quickly edit scripts and HTML code, or system configuration files.
TextEdit is not only useful for quickly editing code (which is primarily stored in basic text formats), but also can be used as to take notes using its rich text format support that is still a very lightweight and common file format, but supports some basic features like text styles, differing font sizes, and lists that are useful for notes and other idea organization.
Overall, at its roots TextEdit is a fairly bland program without many bells and whistles, and is attractive as such for making quick edits and other basic organization where you do not wish for auto-formatting and other details that are common in programs like Word and Pages. However, even though it is lightweight, Apple has implemented an auto-formatting list feature that does so in ways that can be exceptionally frustrating to deal with.
If you have a rich text document that you are managing with TextEdit, by indenting lines using the Tab key or manually bulleting them with dashes or other symbols, TextEdit may take this as an indication you mean to create a list, and will sometimes reorganize your entire document when it does so. In the case of large documents with pages of text or organized ideas, you might find the program suddenly format all of your text into a muddled mess of indentations and list bullets.
Unfortunately the program does not have any settings in its menus or in its preferences that allow you to adjust this behavior, so if you are experiencing such formatting problems, you may be at a loss for what to do about them.
These formatting errors can be overcome by placing your cursor in special areas where the behavior will not trigger, or by using special copy and "paste matching style" options to avoid auto-formatting, but this can be burdensome to keep in mind and you still might inadvertently trigger a full-document rearrangement.
Luckily, even though there are no apparent options for adjusting automatic list generation, you can revert this behavior in the toolbar of a TextEdit window. To do this, open a TextEdit document (or create a new one) and then locate the Lists button in its toolbar (which looks like three lines with bullets before them). In this menu, choose the "none" option (at the top of the menu) and you should now have turned off any auto-formatting of lists.
One might expect this menu to be used for creating a list from a selection or at the point of the cursor, and then disable when completed; however, this is not the case in TextEdit; as long as a list type is selected in this menu, then the program will interpret tabs, dashes, and other listlike behavior as an intent to create a list, and do it for you.