If you have tabs in the line, have insert-spaces-instead-of-tabs on and hit <tab>, it will add a wrong amount of spaces.

You can do it this way:
1. Create a file containing tabs
2. Enable insert-spaces
3. Hit tab on a line already containing tabs.

For instance, having a tab size of 4, if I have a line containing two tabs and I hit the tab key, it will only add two spaces.

This is due to that line: (in gtksourceview.c, 
gtk_source_view_key_press_event) 

> num_of_equivalent_spaces = tabs_size - (cur_pos % tabs_size);

It could be replaced with something like this one:

> gunichar c;
> GtkTextIter tabiter = cur;
> gint tabpos = cur_pos;
> 
> /* Find the position of the last tab of the line
> (the 0 offset if there is no tab) */
> do
> {
>         tabpos--;
>         gtk_text_iter_backward_char (&tabiter);
>         c = gtk_text_iter_get_char (&tabiter);
> }
> while (tabpos > 0 && c != '\t');
> 
> if (c == '\t')
>         tabpos++;
> 
> /* count chars from the last tab to the current position */
> num_of_equivalent_spaces = tabs_size - (cur_pos - tabpos) % tabs_size;

Sorry for the lack of real patch but I'm not able to test it right now. I did this patch on an already patched version of gsv and didn't take the time to redo it against a clean cvs version.