The bug was reported at https://bugs.launchpad.net/ubuntu/+source/gedit/+bug/505765:

Hi,

I was editing a text file (https://bugs.launchpad.net/ubuntu/+source/gedit/+bug/505765/+attachment/1094827/+files/sqlitetest.cpy.rb) when the cursor began to behave strangely. The file contained these lines:

CREATE TABLE people(
 integer primary key,
 name varchar(50),
 age integer
)

When I put the cursor after the 'l' in 'people', the arrow key would not move the cursor to the right. Instead it jumped to the next line. Similarly, I could not add text at this position.

Viewing the file in a hex editor revealed a lot of spaces (284 to be accurate) after the opening parenthesis. Probably I've added those spaces without knowing it (resting my book "beginning Ruby" on the space bar ^ ^). Still, I feel the editor should handle a few hundred chars on a line without misbehaviour.