A single post is destroying my sidebar… Why?

That is actually very common and usually happens because of the html used inside the post. Most of the time it’s because someone forgot to close a <p> tag </p> or link tag or sometimes, by mistake deletes the ending of the image tag “>”. Recheck your most recent post for missing tags.

I see it with all single posts;

caused by the fixed height in:

#sidebar .widget_posts .post_box {
display : none;
border-width : 0 1px 1px;
padding : 5px;
height : 255px;
}

possibly change that to:

min-height : 255px;

btw:
your site (at least the linked single post) has some validation errors:
http://validator.w3.org/check?uri=http%3A%2F%2Ftechpounce.com%2F2012%2F05%2F17%2Fhtc-one-x-evo-4g-lte-delayed-due-to-patent-infringements%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

http://codex.wordpress.org/Validating_a_Website

I’m sure I’ll get no responses on this ridiculous question

not a bad question – just a fundamental formatting issue and not really related to WordPress per se – apart from the fact that the problem happened in a WordPress site.

using Firebug http://getfirebug.com/ or a similar tool helps to investigate these kind of problems.

You do have some unclosed tags </div> as it shows here, so you may consider fixing those codes.