I know the limitation to 256 column isn't a real limitation (http://projects.gnome.org/gnumeric/faq.shtml#5).

But there's still a problem with it :
- create a sheet ;
- fill it with data (and by fill, I really mean it : use all stock) ;
- now add a new column anywhere : your precious data in the last column is now lost.

It is especially harmful to someone adding columns at the start, because that someone may destroy many, many columns before realizing something bad is happening.

Gnumeric shouldn't allow adding new columns when that means it will lose data : the sheet should either be auto-resized or an error message be given.