In a velocity macro, an optional parameter receives an old value even if left empty.

Sorry for messing up the format. Try again:

Reproduce step:
1. Create a velocity macro "lightbox" with two parameters - image(required), width(optional).
!{!{velocity}}
#set($image=$xcontext.macro.params.image)
#set($width=$xcontext.macro.params.width)
!{!{html}}
<img src="$doc.getAttachmentURL($image)" width="$width"/>
!{!

!{!{/velocity}}

2. Write a page with two such macros:
!{!{lightbox img="aaa.jpg" width="300" /}}
!{!{lightbox img="bbb.jpg" /}}

3. Problem: The second lightbox still receive width="300", despite the fact that width is not defined.