Mysterious spaces appear

If I type vbscript.dll (Note: There are no spaces typed.)

It'll show up as:
vb_script_.dll, where _ is space.

Is this a bug or behaviour by design?

If you type it where? In here?

vbscript.dll

Yup, does the same for me, don't know why.

hmm, the page source shows a good old  

Yes, this is an old security feature. The next version of the CMS/forum will no longer doing this.