When an NPC vanishes while you’re not looking, you may just assume he isn’t supposed to be there until you complete some other objective. That’s when the real kicker comes in: once you do know you’ve encountered one of these bugs, it’s possible to get around it by loading an earlier save game - one that’s been saved at some point previous to the disappearance. The problem is, what if you haven’t kept multiple saves, or simply enough multiple saves? Or, let’s say you do have enough saves to go back - you’ll still have to sift through them trying to find the right one, and even then you’ll have lost potentially hours of progress because the NPC disappeared before you completed any number of side missions.
Also note that there are multiple common points in the game where these bugs occur, which means even if you get around one of them, it’s possible to encounter yet another one at some point further down the line.
To give you the gravity of the situation, let’s examine Far Cry 2’s design. The game was designed as an open-world sandbox situation, similar to Grand Theft Auto in that you can take missions in any order you want and have multiple ways to complete objectives. Not only that, but you can choose from one of multiple characters to begin the game with, and the others you don’t choose act as your buddies throughout the campaign. And of course, your choices affect your overall game experience. That’s a hell of a lot of game to fine tune and to give the devs credit, not an easy task at all.
Interestingly enough, the post by Community Developer kimi_ in the official Far Cry 2 forums started November 4, 2008 is titled “Save game freezing? Buddies attacking you? Stuck at xx%? *potential spoilers*”. This thread was started as a way of collecting data and saved games in order to pinpoint the exact issue. Let’s repeat the chain of events so far: the game went gold on October 14, 2008, the thread about freezing issues was started on November 4, 2008. And the press release about the game selling a million copies worldwide went out November 12, 2008. Just several weeks after the game was reported “done”, a thread on the official forums about glitches was created by a Ubisoft employee. This suggests somebody knew about these bugs already.