The Sims is old and fussy and there are a number of common issues which occur when using custom content which can cause the game to crash, lock up or behave oddly. The most common issues and the recommended solutions.
The most common cause of game crashes is a GUID clash between 2 objects resulting from the creator not using their Magic Cookie ID properly:
It is recommended that you purchase a copy of the Sim Enhancer Object Manager ($9.99) which is the only program which properly checks all ID's as well as allowing you to organize your files. It does offer a free 48 hour trial if you simply wish to clean up existing items and then only download from trustworthy sites in the future.
All Parsimonious objects do use a registered Magic Cookie additionally all objects which have ever been published online are always prsent in the game directory when new objects are cloned, so that prviously used GUID's are not inadvertantly re-used. In short, objects you get here should not clash with each other.
Please note that although free object checkers exist, none provide a complete guid check which renders them rather useless if the clash occurs between a couple of objects that are not registered by the program.
The typically happens because there are orphaned .cmx files left in your gamedata/skins folder which do not have any accompanying texures and meshes, or corrupt .cmx or mesh files:
You can use the Skin Doctor Utility to check for orphans:
If an object is removed when switching from Build/Buy to Live mode it is because it requires an Expansion pack you do not have. It will not harm your game, however if you want to use it, you will need to install the required EP.
NPCs (i.e. cashiers, bar tenders, food counter servers etc) that come with custom objects have a selection of body, head and occasionally hand skins. If you did not install these into your gamedata\skins folder then your NPC won't have anything to cover themselves and so will be invisible. This often prsents as a floating pair of hands for NPCs that don't have separate hand texures.
There are 3 causes for Sims being unable to use community objects (which come with service NPC's)