Reverse transcriptase extra bases?

[Eivind Hovig]

Is Superscript completely faithful at the end of a transcript, i.e. will 
the enzyme add extra bases at the end? I experience 4 extra bases on a 
PCR fragment of a size defined by a gene specific 3'transcriptase primer 
and a 5' T7 polymerase sequence (as seen on the DNA level). As far as I 
can find out, the RNA starts being transcribed from base 1 after the T7 
polymerase recognition sequence. Thus, the DNA sequence is well defined, 
and that leaves only the superscript to account for the extra bases, as 
far as I can understand. Suggestions or pointers to obvious, but missed, 
points are appreciated.

Sincerely
Eivind Hovig