drupal - Drupal 6 does not like it when an image tag's absolute path contains "http:"

Question

I'm using Drupal 6.28 and there's something i don't understand.

i have a page that contains an image tag pointing to an absolute path. e.g.

<img alt="google logo" src="http://www.google.com.sg/images/srpr/logo11w.png" style="height:190px; width:538px" />

when the content is saved, drupal returns a "Page Not Found" error.

i found out that if i remove the "http:" at the start of the src attribute, there wouldn't be any problem. everything would be ok. i.e.

<img alt="google logo" src="//www.google.com.sg/images/srpr/logo11w.png" style="height:190px; width:538px" />

is there some setting that i may have messed up that causes this?

Thanks