Double click a RichText component in the designer, one with multiple lines of text.
If you press spacebar when changing the content, weird things happen. Sometimes the cursor jumps elsewhere, sometimes the line returns disappear, and sometimes both.
Workaround is to edit the content in the sidebar instead.
I just stumbled across a similar bug when editing the text of a label. After creating a new line with Shift-Enter, I typed a few letters and then hit the Spacebar, at which point the cursor jumped up a line.