Changes have been made in Git (master):

commit 7cc003c648eea4236aff3eafcc1f7aa42bf8f380
Author: Jan Schneider <jan@horde.org>
Date:   Tue Aug 6 17:26:24 2013 +0200

     [jan] Remove text overlaying menu icon (Bug #12496).

  framework/Core/package.xml |    2 ++
  1 files changed, 2 insertions(+), 0 deletions(-)

http://git.horde.org/horde-git/-/commit/7cc003c648eea4236aff3eafcc1f7aa42bf8f380

Changes have been made in Git (master):

commit 062efef1af23eb8264814f52ef340fb893c2851a
Author: Jan Schneider <jan@horde.org>
Date:   Tue Aug 6 17:24:40 2013 +0200

     Don't set alternate text (Bug #12496).

     Give up on supporting broken rendering engines of a browser 
that's not supposed
     to use this interface anway (Bug #11937).

  framework/Core/lib/Horde/Core/Topbar.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/062efef1af23eb8264814f52ef340fb893c2851a

Re-opening and assigning to Jan to look at when he gets back.

There are definitely some concerns regarding making formatting changes 
in a package that break the existing layout (as opposed to changes 
that fix the current issue without breaking the existing layout).

The fix you posted works for me as well

I found a change to horde/themes/default/screen.css at the bottom of 
the github.com link from  Michael Rubinsky below in this bug report. I 
assume that may be the css change necessary. I did not have that css 
in my screen.css (This is a default horde 5.1.2 install from pear). I 
put it in (cleared any cached css) and it did not work. However, 
copying the text-indent: -10000px; line from the selector 
".horde-icon-settings a" to just ".horde-icon-settings" did fix the 
issue.  The "Settings" alternative text is no longer visible.

To be clear, here is what that part of my 
horde/themes/default/screen.css looks like now:

.horde-icon-settings {
     background-image: url("graphics/settings.png");
     background-repeat: no-repeat;
     text-indent: -10000px;
}

.horde-icon-settings:hover {
     background-image: url("graphics/settings-active.png");
}

.horde-icon-settings a {
     width: 22px;
     height: 27px;
     text-indent: -10000px;
}

I see the same problem in a fresh install of Horde 5.1.2. I had no 
stale CSS to clear. Could you link to the CSS change in github that 
fixed this?

For me, clearing the browser cache doesn't fix this problem.

Horde version: git master

I must be careful in future with pear upgrade, but how do I know which 
package I can update?

No, that is incorrect.  *This* is stale CSS - I saw the same thing and 
cleared my CSS cache and it was fixed.

His comment deals with some browsers where the gear icon will be 
displayed and the text will appear AFTER the icon.  If the text is 
appearing UNDERNEATH the icon, the CSS is not updated.

Although this could get into a discussion of how to handle these kind 
of fixes... since it is possible to run a version of Horde_Core 
without requiring upgrading Horde (with the necessary CSS updates).   
This is a limitation of the way we distribute our applications/packages.

Jan's comment on the mentioned ticked suggests it's not stale CSS:

http://bugs.horde.org/ticket/?searchfield=11937&x=0&y=0#c10

sorry forgot to upload the attachment.

You haven't updated your CSS.  Or you need the latest version of 
Horde, which contains the CSS changes.

Probably caused by:

https://github.com/horde/horde/commit/3bb702b55f2783084ad97a2a8e23e41337d07b59#framework/Core

Where? What icon? Doing what?

After update of Horde_Core to 2.6.3 the word "Settings" is over the icon.
Downgrade to Horde_Core 2.6.2 solve the problem.