159 wanted posters. When I saw this achievement I said "Fuck that!", but two weeks later I caved and decided to do it. It wasn't fun, but it's done. 
 
Or at least I thought it was done. After the fucking hours it took me to drive around and collect every goddamn poster, the game glitches on me somehow and says that I have 158/159 posters. I thought to myself "Oh, I must have missed one somehow", so I went to view what I was missing.
 
Poster #106 apparently. I look it up online, see the spot and I noticed that it looked familiar. Sure enough it was familiar because I had already gotten the motherfucking poster and it isn't counting it. 
 
I want my S Rank...help me.

I'm sorry, I can't help you. I can show you this though:

SO unlucky. Can't help you either unfortunately. But I do have plenty more cat pictures if you'd like to see them. 

Fat cat. I hope this makes you feel better. :P 
 

Ouch man. If it helps, you have 158 more than me... I havent even found one poster yet?

" I'm sorry, I can't help you. I can show you this though:

"

" @JJWeatherman said:

" I'm sorry, I can't help you. I can show you this though:

"

Now I don't know what to believe. "

So im not the only one.. But for me it is 8 (!) Posters left, and ive already taken them but cant see them in the menu. I should've already had the trophy, its so frustrating.. I have 98 % of the game finished so thats the only thing that keeps me from 100 %.. Fuck.......

"  I'm just staring at a 99% completion wondering if my whole existence is futile... think they'll patch it before tomorrow's DLC? "
 
I sure fucking hope so >:(

Are you getting Jimmy's Vendetta? I think it looks just like The Betrayal Of Jimmy..

Ohhhhh, that sucks. Hope they get it fixed after all of that work.

I'm sure there's a valuable lesson to be learned here.

I beat the game without finding a single wanted poster. 
 
I think I'll go ahead and consider that a win for me.

" I'm sure there's a valuable lesson to be learned here. "

Okay, I think i'll do it too because ive waited so long for this game (loved Mafia 1) but hey just let me know if it get fixed about the wanted poster for you :)

" @Jimbo said:

" I'm sure there's a valuable lesson to be learned here. "

Yeah but people would only learn it if it unlocked an achievement. "

I hate damn collectable achievements.  
 
I'm sharing your pain with my kitten avatar.

You might be having the same issue I had. Game said I was missing #48 and all of the guides pointed me to places I had already been. Apparently a lot of them are numbered differently for whatever the reason. It took several and doing a lot of checking before I finally found the right location for it.

Had the "same" problem but in another game.
 
In Just Cause 2 there is 369 settlements and i finished 368 of them. I was trying for days and days to finish the last settlement but found out it had a glitch with a watertower or something. So i had just wasted days and days and days and days and days to finish the 368 settlements and days upon days on finishing the last settlement wich in the end turned out to be fucked up. So it's impossible to get 100% completion.
 
Retarded developers :)
 
Things like that can damage the overall impression of the game, giving you an unsatisfied feeling when ejecting the disc from your console.

Achievement unlocked! So what happened was...the MULTIPLE guides I looked at online all had the wrong location for the real poster that I had actually skipped. Word of advice if this happens: Look at the name of the poster you are missing. All posters in the same row have the same first name with the last names in alphabetical order. Man am I happy.
 
Big thanks to DiscoDuck8k!