This bug is marked fixed, but it has not been fixed. 
Jason marked it as fixed on 2008-05-08, with this note: 

  I've marked this as fixed, since the bug reported will 
  be fixed when font-backend is merged. 
  I will not close it at this time though, as valid points 
  about documentation were raised. 

Then, on 2008-06-12, I received a mail saying that it was 
closed. In any case, it is *not* fixed - I see the same 
thing in this recent build: 

GNU Emacs 23.0.60.1 (i386-mingw-nt5.1.2600) 
 of 2008-08-01 on LENNART-69DE564 
Windowing system distributor `Microsoft Corp.', version 5.1.2600 
configured using `configure --with-gcc (3.4) --no-opt --cflags 
 -Ic:/g/include -fno-crossjumping' 

There are two things that might be the problem: 

(1) `modify-frame-parameters' changes the `font' parameter behind your 
back from the value you provide `modify-frame-parameters': 

(modify-frame-parameters frame 
  (list (cons 'font "-outline-Lucida Console-normal-normal-\ 
normal-mono-15-*-*-*-c-*-iso8859-1"))) 

The `font' frame parameter is then: 

"-outline-Lucida Console-normal-normal-normal-mono-15-\ 
*-*-*-c-*-fontset-auto8" 

IOW, iso8859-1 gets replaced by fontset-auto8. 

(2) `x-list-fonts' returns nil when passed such a font (i.e. with 
fontset-auto8). 

This breaks my code. Though the frame and the font appear normal, 
`x-list-fonts' does not recognize such a font. My code changes the 
font name to use a different size (e.g. changes 15 to 14), but it 
checks that `x-list-fonts' recognizes the font name before trying to 
use it. And `x-list-fonts' does not recognize the name (with 
"fontset-auto8") that `modify-frame-parameters' establishes behind the 
scene. 

The font that I provide to `modify-frame-parameters' is recognized by 
`x-list-fonts', and it has the same appearance, but it never appears 
as the frame parameter in this context because 
`modify-frame-parameters' substitutes a different name. 


Below is the pertinent part of the original bug report. The symptom is 
the same, but the font that `modify-frame-parameters' substitutes is 
slightly different. 

Back in April, it substituted: 
"-outline-lucida console-normal-roman-normal-mono-15-\ 
*-*-*-*-*-fontset-startup" 

Now it substitutes: 
"-outline-Lucida Console-normal-normal-normal-mono-15-\ 
*-*-*-c-*-fontset-auto8" 

Neither is recognized by `x-list-fonts'. 

--------8<---------2008-04-05 report ------------------- 

(frame-parameter nil 'font) -> 
"-*-Lucida Console-normal-r-*-*-14-*-96-96-c-*-iso8859-1" 

(modify-frame-parameters 
 nil 
 (list 
  (cons 
   'font 
   "-*-Lucida Console-normal-r-*-*-15-*-96-96-c-*-iso8859-1"))) 

(frame-parameter nil 'font) -> 
"-outline-lucida console-normal-roman-normal-mono-15-*-*-*-*-*-fontset-startup" 

What's that about? In Emacs 20, 21, and 22, the result is just the font I 
specified. 

I have code that zooms frames (font size). I change just the point size in the 
font spec, using `x-decompose-font-name' and `x-compose-font-name'. I check that

the result is a legitimate font using `x-list-fonts'. If not, I increase or 
decrease the increment until I find the font that works with the closest size. 

[Yes, I know there are other ways to adjust font size, but I've found that this 
method is flexible for users and provides certain benefits.] 

My code no longer works without change, because after one call to 
`modify-frame-parameters' the font is no longer something recognized by 
`x-list-fonts'. I can comment out the part that iterates until it finds a size 
that works (recognized by `x-list-fonts'). That works, but I'm still curious 
about this. (Is there perhaps a bug in `x-list-fonts' or in 
`modify-frame-parameters'?)