Steps to replicate:
Launch Bitcoin 0.5.2 Use the Send coins tab Type a label for the address you intend to add Paste the Pay To: address See label disappear Type a new label Type another character in the address field See label disappear
One should be able to construct a transaction without the GUI second-guessing and wiping what you just typed in.
If for some reason this was constructed as desired behaviour, an alternative would be to disable label input until a valid bitcoin address is entered into to Pay To: field.
This is the case because entering an address in Pay to: looks up the address in the address book, and fills it in the associated label field. If nothing found, it clears it.
Maybe it should only fill in the label if the label field is still empty.