Using CTRL-X (cut) in the textbox text property causes the control to be cut/deleted.
1) Create a text control
2) Type something in the text property
3) Highlight the contents of the text property
4) Press CTRL-X
5) The control gets removed
This occurred with:
Eclipse 2.1.2
Java 1.4_x
Windows XP
SWT Designer Pro 1.2.3
Just another bug report for you.