Subversion's design is made up so that you will never ever lose any data. But there's one command that will do exactly that: Revert.
Reverting means discarding all modifications you've made to one or more files which you haven't committed yet.

But what if you selected the wrong file(s) to revert? Sure, TortoiseSVN always asks you first before it reverts the files. But as we all know, such "Are you sure?" dialogs are too easily dismissed by clicking "Yes".

So what if you reverted files and you then discover that you shouldn't have done that?
If you used TortoiseSVN to revert, you can get the reverted changes back again! Because TortoiseSVN doesn't just revert your files but first moves them to the Windows recycle bin (trash folder).

That means your changes aren't lost completely: you can find the files in the Windows recycle bin and restore them from there.