[R] strange behavior, adds new field by non-existent field

[Peter Keller]

data:
> tmp1
      Date HrMn  Temp Q.4
1 19450101 0900 -37.0   1
2 19450101 1000 -35.9   2
3 19450101 1100 -35.9   3
4 19450101 1200 -36.4   4
5 19450101 1300 -36.4   5
6 19450101 1400 -36.4   6
7 19450101 1500 -36.4   7
8 19450101 1600 -37.5   9

Accidentally, I did this (I meant to write Q.4 instead of Q here)
> tmp1$Q[tmp1$Q!="1" & tmp1$Q!= "5"]<-NA

I would have expected it to tank, but still got a new field with the the
values I wanted
> tmp1
      Date HrMn  Temp Q.4    Q
1 19450101 0900 -37.0   1    1
2 19450101 1000 -35.9   2 <NA>
3 19450101 1100 -35.9   3 <NA>
4 19450101 1200 -36.4   4 <NA>
5 19450101 1300 -36.4   5    5
6 19450101 1400 -36.4   6 <NA>
7 19450101 1500 -36.4   7 <NA>
8 19450101 1600 -37.5   9 <NA>

Can someone explain this?
Peter

> sessionInfo()
R version 2.10.1 Patched (2010-03-16 r51294) 
i386-pc-mingw32 

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] lattice_0.18-3 fortunes_1.3-7 zoo_1.6-2     

loaded via a namespace (and not attached):
[1] grid_2.10.1  tools_2.10.1

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