You may have written something like
	wish: set foo bar
	wish: if {$foo == bar} {puts stdout bar}
	syntax error in expression "$foo == bar"

in which bar is interpreted as neither a string nor a variable, since
strings as operands in expressions MUST be surrounded by double quotes
or braces. 

Change to
	wish: if {$foo == "bar"} {puts stdout bar}
or
	wish: if {$foo == {bar}} {puts stdout bar}

always in expressions, depending on if you want expansion performed or
not. 

Contributed by "Jesper Blommaskog"