Were they long lines that were wrapped?
:set nu list
1 one one one one one one one one
one one one one one one one one$
2 two two two two two two two two
two two two two two two two two$
Using a visual-block selection at the beginning of each line and
using insert at the beginning (`:help v_I`) will put them at the
beginning of each actual line, not the second (wrapped) part of each
line.
There are other ways to insert on alternate lines, but they tend to
require much more intentionality and you'd know you were doing it. :-)
-tim