oligo mutagenesis

[Elaine M. Weidenhammer]

Recently, two people in our lab have attempted HA-tagging using oligos;
a third person is undertaking site-directed mutagenesis with an oligo. 
In at least two of the three cases,  sequencing revealed mutations
(insertions, rearrangements) in portions of the gene a considerable
distance downstream of the site complementary to the oligo.
Has anyone else had these difficulties?  Does the source of the oligo
make a difference? (i.e. a professional company vs. a university "part
time" facility)

Any comments are greatly appreciated!
Elaine Weidenhammer