Introduction:
This issue seems to have been commented on a bunch on other boards
but no resolution was ever noted. I don't know if this is the best
solution but it got me back on the road again.

Setup:
Current user: Full Control permissions
Site Collection Administrator: No
Logged on to top-level collection
SharePoint Designer(SPD) also open to top-level site

Purpose:
Testing features to identify which are available based on user roles
and their respective permissions

Issue:
Performed the "sign in as another user" function; logged in
as "Contributor"; did not log out
Performed the "sign in as another user" function; logged in
as "Reader"; logged out

I then switched to the SPD session to edit the master page; saved
the file but received the error dialog: "Server error: access
denied". I also noticed I could not see all the options at the top-
level collection via Site Actions / Site Settings / Modify All Site
Settings.

Resolution:
1) I took my permissions off all user groups [basically, I will
start fresh with new permissions]
2) My cohort updated my permissions to the respective user groups so
that I again had "full control"; SPD still failed and all the
options still did not show at the top-level collection via site
settings ...
3) My cohort went into Central Administration / Application
Management and added me to the Site Collection Administrators list

That worked.

Hopefully this will help others; researching and debugging this
issue took 2-3 hrs.

Other Options
If you have more information on this issue with better resolution
steps, please post a reply and email me directly so that I can be
sure to get your information.