Mitsubishi GX Developer "strange behavior"

Hi,
This is my first project using Mitsibishi's PLC.
I'm programming an FX1N using GX Developer Version 8.12N.
Everything worked fine programming,dowloading to PLC,running the
machine,etc.
After a couple days I wanted to do some modifications in the ladder but when I tried to change timer numbers ,for example T3 to T4 the GX Developer ,instead of changing the timer number,add a new line to the ladder like this:

x0
-[ ]---------+--(T3)
..................+--(T4)

And it is difficult if not impossible to delete the T3 line and
replace it with T4 to obtain this:

x0
-[ ]-----------(T4)

The same happens if I try to modify other device numbers,it leaves the old one and add a new one.

After a while I decided to delete the whole ladder and write only
one rung but it still behaves in same way.
Then I decided to do some tests as follows:

1. Reinstall the GX Developer.Doesn't solve the problem.
2. Open a new project and write some rungs.In this case everything is working correctly.
3. Open a new project in one instance of the GX Developer and the
"problematic" project in a second instance of the GX.Then copy and paste all the ladder from the "problematic" to the new project.In this case everything is working correctly.

My conclusion is that my code is correct and also the GX Developer is working OK.But something is wrong in the "problematic" project.

Temporary I solved the problem but I don't understand what
happend and I'm afraid it could happen again.

Did anybody have such a problem with Mitsubishi's GX8?

I'm attaching a zipped "problematic" proyect(with only one rung) and a Word file with snapshots of the steps I explained above.

Shaul

This not strange, it is normal behaviour (read the GX manual).

What you have seen is so called INSERT mode and what you want is OVERWRITE mode. You can toggle between them by pressing INS key on your keyboard.

Thank you Panic Mode, you are right,it was in INSERT MODE.