I'm not sure what changed, but lately Vim has been driving me nuts. Whenever I try to do a column mode insert, vim takes my current character and adds to the last character I inserted. For example, the first time I do a block comment by inserting # on multiple lines, it works fine. The next time, however, I end up with ## inserted on every line, and the problem just compounds from there.
To do this, I'm hitting Ctrl-V, down or up arrow, Shift-I, #, and then Esc. This worked for months, but now it seems to be pasting extra stuff in. I've tried disabling all .vimrc files, but the behavior remains the same.