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[Caption inside an oblong panel:] (An unmatched left parenthesis creates an unresolved tension that will stay with you all day.

In programming, punctuation is often used to mark sections of code. Paired punctuation marks must always be matched up with a corresponding closing mark, otherwise a so-called syntax error occurs. The programming language Lisp (also featured in 224: Lisp is known for large numbers of nested/paired parentheses. Even in literary works intended only for human consumption, the absence of a matching closing parenthesis as appears in this sentence or other "balanced" punctuation sets creates a mental expectation of eventual closure and completion that remains unfulfilled even long after the unmatched mark is encountered.
There is also reference to 312: With Apologies to Robert Frost which could contain the missing parentheses.
It can also be interpreted as a metaphor , which compares the reader with a Lisp interpreter . The interpreter looks for the parenthesis until the end of the file, where it eventually halts, and prints out the error. The comic claims that if you read an unmatched parenthesis, you will look for it for the rest of the day too.
It also refers to an awkward feeling when you see something out of place in a piece of literary text (like unmatched parentheses, spellying error or a randomly-plac,ed comma..
Finally, in some countries (Russia in particular) they use just parentheses instead of text smileys so that :) turns into ) and :( becomes (. Hence Russians can magically resist the unresolved tension of the comic but may feel a bit sad instead as a side effect.
The title text refers to the same issue as already highlighted in 327: Exploits of a Mom : if your scripts trust external input, you sometimes will be surprised. At the time of this comic, there were quite a few websites that would grab the  xkcd comic three times a week and publish them on their own site. This comic likely broke at least some of the websites because of either the unmatched brace or the extra unmatched markup that is in the title text.
[Caption inside an oblong panel:] (An unmatched left parenthesis creates an unresolved tension that will stay with you all day.
