In the formatted preview for Questions & Answers, typing a tag link with an invalid tag name "crashes" the preview. As soon as you type the following the preview stops updating and an error can be observed in the Javascript console.
[tag:+]
Observed in Safari 7 and Chrome 31.
The + character in tag names breaks links. seems related to the business rule of not allowing certain characters in tags, but it sounds like a different bug.
[tag:.]
(period), and I'm presuming other characters as well. Escaping the braces/colon does not stop the problem from occurring. I just ran into this issue as well and was directed here.[tag:and]
.[tag:or]
.