Depending on which board you're in when you click on a global post, it affects whether the post will be locked, and what options will be present when editing a post.
Some examples:
Announcement will be locked from replies wrote:1). Go to the Cognitive Dissonance Forum
2). Click on a Global Announcement
3). Thread is locked! Can't reply.
Take-home message: If you can't reply to a global announcement, check which board you're in at the upper nav bar. If you're in Cognitive Dissonance (or any locked board)... well, it won't work.Announcement will NOT be locked from replies wrote:1). Go to a normal forum (like Forum/Wiki Feedback)
2). Click on a Global Announcement
3). Thread is unlocked! Yay!
---Most of those Special Recent Post Pages Do It Too wrote:1). Go to forums home page
2). Click on View Active Topics (or New Topics)
3). Click on a Global Announcement
4). Thread is locked!
5). Look at the board. You're in Cognitive Dissonance! Nuuuuuh!
It also applies to poll options and whether a post can be made into a global post or not (this depends more particularly on your usergroup powers).
For example, on my account, I can't make a poll in News, Forum/Wiki Feedback, or Unified Format guidelines. If I go to those boards and try and edit my first post, I can't edit the poll! However, if I access the announcement from anywhere else, I can make poll changes.

Additionally, I can't make any threads global/announcement/sticky from the Administrative Forum, so I if I try to access my announcement from there, I can't un-announce the thread. However, if I go from any other board, I have the powers to do so.
One final curiosity: I don't have merge/split/move post privileges... but there's actually quite an interesting workaround I can do.
@_____@;;1). Turn the thread into a global announcement
2). Un-announce it, and I can place it into whatever board I want...
hmmm....