I use Tcl for a number of analysis scripts inside some scientific software. Recently, I came across a situation in which I had nested loops:

    for {set i 0} {$i<$n} {incr i} {
        # Some comment
        for {set j 0} {$j<$n} {incr j} {

The script refused to run and eventually I figured out that the comment was somehow impeding execution. I still don't understand why.