[imgs=https://scontent-b-ams.xx.fbcdn.net/hphotos-xpf1/t1.0-9/q71/s720x720/10447406_10152582771569878_399879017678674370 _n.jpg]And back at the 'local'![/imgs]

There is a space that is added between the 1 and the 0 just after the first underscore...
It's not in the original URL, but keeps being added when the image is linked to.

The issue can be seen there

Weird!

Yup. Long strings are split with spaces.

Sorry, that makes no sense at all...

Most such links work perfectly.
Juts the odd one or two have additional 'spaces' added - nothing to do with underscores at all...

You can't add quotes and apostrophes as the altering text for the picture. Try to remove quotes, and the BB code should work.
This is a security issue we still have no exact solution about, and I'm experiencing kind of discomfort regarding this often, too.
Hopefully, in the future I'll find a solution to fix it.

That was it!

A very strange bug indeed!!!

It's not a bug, it's made intentionally.
Providing quotes and apostrophes in ALTs could lead to security leak.

It's mentioned in updates history - Changes comparision: 2.5 - 2.5a [released: November 5, 2010].

Hm, I may have a solution. Let quotes / apostrophes in file names be replaced with the 'grave accent' sign, ie: `. It looks like apostrophe and should not create a security issue.

It's not easy as you might think, couldn't be achieved with a regular expression of just one line...