http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/tell.htm 

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'''tell and flush'''

If you are using tell and you do reads and writes in between your calls to tell, 
you usually will have to do a flush or your tell will fail with a -1 return value.
 
This is because the Tcl Channel code cannot ensure an accurate 'tell' value
until you flush out pending i/o on your open channel. -mikeH

''need some examples of this behaviour here...''

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'''tell and open using "a", "a+", APPEND , etc..'''

If you are using tell in combination with a channel opened via APPEND (et. al), 
keep in mind that for each write performed, your file position is 
going to be placed at the end of the file. Tcl Channels are reflecting C library
behaviour in this respect (which it should do if you think about it). -mikeH

''need some examples of this behaviour here...''

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