Sometimes, I didn't remember what keys had I pressed, occasionally switched a text box to right-to-left layout. Well, maybe this is a very useful feature for Arabic users, but I just don't know how to switch back to left-to-right layout.
(P.S. In MS Windows, there's a text direction option in the right button menu for the text box. However, in Linux (GNOME Classic) I didn't see any.)
In GNOME (v2 and v3) the direction of a widget is set by the first character in the text box. That means that if the text you insert into the text box starts with a unicode character that has RTL directionality, the layout for the rest of the content will be RTL.
If you want to override this behavior you can insert an invisible character called RLM/LRM as the first character in your content. In GNOME this is done by right click->"Insert unicode control character"-> select RLM or LRM. keep in mind that once it's used, RLM/LRM becomes part of your string and always be displayed that way.
There is an unofficial GNOME input method that makes it easier to manage these characters, but that will probably be used by RTL users who need it more often.