automatic completion of object names

Hello there,

I just upgraded to R 2.15 (from R 2.12) on a Windows XP machine and noticed some puzzling behaviour (that in my opinion did not exist in R 2.12).

It is possible now to call objects without spelling out the full object name. R now seems to use that (unique) object which shares the same beginning of the called object, even though the originally called object might not even exist. 

This is quite awkwardly described, but perhaps you know what I'm talking about?

Let's say I'm using the Mroz data supplied with the car package:

require(car)
data(Mroz)

there is no variable called "w" in that dataframe, but calling 

summary(Mroz$w) #### Mroz$w does not exist

does not return any error but instead gives the same result as 

summary(Mroz$wc) ##### exists in Mroz


I find this behaviour *very* undesirable. Is there any way to switch it off?

Best,
Irene