Hi,

Apparently, I don't seem to be able to reproduce the issue since the Add method works very well even with you example (I have opened a new project and pasted in your code - after adding a TabControl to the form).
However, since you just pasted a short piece of code, please provide us the full scenario including VS version, SDK version and all other environmental parameters you can collect.

A small sample application will be much appreciated as well...

Regards,
Itzik Spitzen