This is a term meaning ''unexpected side effect, behavior, consequence, requirement, etc.''

You frequently run into these in the worst possible moment.

For example, a Tcl ''gotcha'' is the (at least pre-8.5) Tcl treatment of
numbers with leading 0. While there is a perfectly understandable reason
for 010 meaning 8, if someone was writing code expecting to
generate numbers from 000 to 999, they would experience gotchas with 008 ,
which isn't a valid octal number...

Another Tcl '''gotcha''' is to hand arbitrary strings, read from the user or a file/command, directly into a list operation without first ensuring that the contents is, in actuality, a [list].


I don't intend the above as a complaint about Tcl. I intend it as an example
of a ''gotcha'' (at least to someone brand new to Tcl... once you hit that
one, you generally become paranoid about it...)

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[RS] One possible gotcha is [switch] -- always use "--" before the switch variable, since if the value of the switch variable starts with "-", you'll get a syntax error.

[KPV] Also, [comment]s w/i [switch], while possible, are tricky.
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[RS] 2010-02-24: Yet another gotcha we ran into last night: Consider a function
 proc f x {
    if {$x == "00000000"} {
        puts "$x is NULL"
    }
 }
which reported
 0E123456 is NULL
How so? Bug? No -- feature. With the [==] comparison operator, the operands are tried to match as integers, floats, or then as strings. And the $x in this case, though meant as a pointer in hex, could be parsed as float - with the numeric value of 0, which is numerically equivalent to 00000000. The solution of course was to use the [eq] operator instead.

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!!!!!!
%| [Category Glossary] |%
!!!!!!