by emiel.mols:

What steps will reproduce the problem?
Two test cases which evaluate differently can be found here
<http://gist.github.com/410574>;. I 
use 8g version 5440.

What is the expected output? What do you see instead?
The compiler to generate code that either first evaluates all expressions in the chained 
arguments, or code that evaluates the chain from left-to-right. Either way, not to do it 
differently. 

What is your $GOOS?  $GOARCH?
darwin; 386

Which revision are you using?  (hg identify)
17ded5ad443b tip

Please provide any additional information below.
Relevant assembly is also attached. Line 27 clearly starts with invoking s.Len(), while
line 33 
begins with s.Clear().

Btw, I might imagine that this is just 'the way things work' with the len(...)
expression probably 
just being a pointer to the slice's length field or something, but still, from an
implementing view it 
took me half the night to figure out what the hell was going on.