This is a hash-bang: #!
Hash-bangs cause havoc or a number of reasons, but there is at least one example that directly affects how Collapse-O-Matic and Collapse-Pro-Matic process URLs. Take external triggers, for example:
[expand title="question 1" id="q1"]answer 1[/expand]
[expand title="question 2" id="q2"]answer 2[/expand]
<a class="expandanchor" href="#q1">expand question 1</a>
<a class="expandanchor" href="https://spacedonkey.de/2207/collapse-pro-matic-pesky-hash-bang-urls/#q2">
question 1
answer 1
question 2
answer 2
External link to expand question 2 from anywhere.
So far, so good. But now, we add a link with a hash-bang:
<a class="expandanchor" href="https://spacedonkey.de/2207/collapse-pro-matic-pesky-hash-bang-urls/#!/monkey/wrench/#q2">answer 2 with a hash-bang monkey-wrench</a>