http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/unset.htm 
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Note that you should use a [catch] with unset if there is a chance that
a variable being unset hasn't been created yet.

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Did you know that an unset can appear to fail?  [Erik Leunissen] wrote
about this on comp.lang.tcl during Oct, 2002:
It is an error for the variables to not already exist, the name to be illegal,
Well, I've been bitten by this one on a *regular* basis, but not on a 
*frequent* basis. So infrequently, that I tend to forget about the 
issue, and once it happens again, it takes me by surprise and I need 
hours to find the cause of evil, which I'll explain:
A [trace] on the variable can resurrect it.
The following happens when you bind a variable to an entry widget 
through the -textvariable option and subsequently issue [[unset 
textvariable] :
[DKF]: In 8.6, `[unset]` is bytecode compiled. (And don't use `catch
   1. the textvariable may become unset for a fraction of a millisecond, I'm not sure. Perhaps somebody else can elaborate on this ...
   2. the textvariable binding will immediately reassign the value of the contents of the entry widget to the textvariable. (If it has been unset for a short moment, it is recreated)
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So what's the recipe then?
In retrospect, `unset` should have been designed so that it was not an error to
   1. release the binding to the entry widget by:
   * doing [[ .entry -textvariable {} ], or
   * destroying the widget entirely
   2. unset the variable: [[unset textvariable]
   3. try to remember this issue (better than I do) when it takes you by surprise the next time.

For the coming releases of Tcl (still in CVS), you can also set an unset 
trace on the textvariable, and let the trace handler do the above step 
one and/or two. For current releases this will not yet work because of a 
(recently fixed) bug.

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[TV] 24 apr '03
I just found behaviour I didn't get:

 (Tcl) 68 % info var http::*
 ::http::urlTypes ::http::http ::http::1 ::http::alphanumeric ::http::encodings ::http::formMap ::http::defaultCharset
 (Tcl) 68 % unset ::http::1
 can't unset "::http::1": no such variable
 (Tcl) 69 % info var http::*
 ::http::urlTypes ::http::http ::http::alphanumeric ::http::encodings ::http::formMap ::http::defaultCharset
proc test {n cmd} {
Its wish 8.4.1, and it runs bwise, a webserver (tclhttpd with some alterations), and this is clearly from the http package to fetch a webpage. Maybe the manual gives a neat answer, I just found it noteworthy that a unset in error still seems to do it's unsetting.

[RS]: ..or that the variable was removed by the web server between the first two commands? What happens if you just call the first command repeatedly?
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