EDIT November 8th: there is a very good reason I shouldn’t post at 2:00 am; especially when I’m practically asleep1. Hopefully, this way it’s clearer. Screenshots were created by me using photoshop (they are not from the actual website).
The website I am working on, has a lot of submission forms, where various data is being input, as well as validated and then - if correct - processed using ASP.NET. Seeing as we have a deadline shortly, debugging was high on the list. As ‘luck‘ would have it, crashes seemed to get weirder by the hour. “I swear it was working properly yesterday” could very well become our motto soon.
But back to the bug in question: a co-worker (a.k.a cw ’cause I’m lazy), demonstrated his amazing bug find.
Step1: introduce an invalid file type (in this case a fictional one without extension). Error message reads: Invalid file type. Please select another one.

Step2: keep typing away. You will eventually notice that user-editable areas are larger than they might initially appear.

Step3.1: Start removing evidence of your weird achievement. Everything normal so far.

Step3.2: Keep removing the text. Not normal anymore, but it’s a handy feature2 nonetheless.

Step3.3: You’re still pressing the delete key. This feature is not quite what you had in mind, but might as well see what it does.

Step3.4: Deleting away… even though you shouldn’t be able to.

Step3.5: Pressing delete once again. On second thought it’s best to hide all traces. Popularly known as “WTF??“.

Summed up: basic submission form with an error message accessed from Internet Explorer 7 (and 6). Repeated introduction of invalid data is apparently possible to otherwise non user-editable areas (i.e. inside a span tag). And repeated deletion deletes the span tag, and a file upload input control.
I later found out that said cw managed to fix the problem:
Me: By the way CW, did you check it in Firefox as well?
CW: Yes. It doesn’t happen. But I fixed it.
Me(impressed): Really? What was the problem?
CW: I don’t know, but I can avoid it.
Since idiot-proof programs(websites) are no match for idiots (the latter being the better), the conclusion is clear. Feel free to draw it yourself
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1 bet none of you can sleeptype: sleepwalking is overrated.
2 there are no bugs, only misunderstood features.