Important
As of September 2020, this guide is probably no longer applicable due to the new update. I don't have this game on my disk atm but I'll let you guys know if there's a workaround asap, *sigh in Hunter growls*.

Recently I returned to L4D2 to play with friends who just got the game during Christmas giveaway, it is still a nice experience, abeit some newly found bugs, like the "Model not precached" CTD errors while hosting local server on L4D1 maps, however I've figured how to fix them, here it is:

The Bug
If you've installed certain addons (either via Steam workshop or downloaded from web), when you host a local server on L4D1 maps (namely: The Sacrifice, Dead Air, etc), after a new level is loaded (usually the 2nd level), your game will crash to desktop with the error message "... UTIL_SetModel: not precached: model/xxx..."(the xxx can be a survivor model or infected model, it doesn't matter)poping up, and all players in your server will be disconnected or told "lost connection".

The Cause
According to discussions on l4dmaps.com, some mods (e.g. a custom campaign or a custom model) use an older .txt list of files to be precached, but since the game has updated, the list is no longer the same, if you load the game with the said mods, some models might not be precached, thus the error message pops up. This is especially prevalent on L4D1 maps ported to L4D2, where the game uses a separate file list.

The Fix
1. If you have encountered this error, make sure to verify your game cache first (right click on L4D2--properties--local files--verify integrity of game cache), it may not be necessary, but can resolve other problems this guide cannot fix.

2. If you have less than 5 addons installed, you may better try disabling them (untick the box before the addon name in game menu) and enabling them one by one, in this way you can find out which mod has caused the problem, then you can nicely inform the mod's author that his/her mod needs updating.

3. If you have installed more than 10 addons (like myself with 20+skins and 10+ campaigns), the above No.2 solution should be a headache. Don't worry, you just need simply put the following text " +precache_all_survivors 1"(without the quotes) in your game lauch options (properties--set launch options), you can use this together with other launch options, like this: "-heapsize 1572864 -nojoy -high +precache_all_survivors 1".
Alternatively, you may add the line "precache_all_survivors 1"(without the "+") to your autoexec.cfg (loacted in \Steam\steamapps\common\left 4 dead 2\left4dead2\cfg), it has the same effect as launch options.

Et voilà! The errors are gone, you can now host local server on all maps without the annoying CTD. Have fun with the game and happy 2014:D