Unfortunately there's no good way of predicting when this happens, the best advice I can give is to use the longer life span settings because it seems to happen after a fairly small number of generations rather than after a specific time.

Population bugs - it varies from user to user, but frequently too many Sims in a world will cause the game to fall over catastrophically.

The Late Night and Show Time building shells often develop faults with the doors which can frequently cause Sims to be locked out of a building - the only solution is to delete the lot and put something else in its place.

Lots being inaccessible - world decor can interfere with Sims being able to get into a particular lot, as can excess non-routing paint (which is supposed to tell Sims where they can't go, but sometimes an extra splash in the wrong place can be a total pain).

There are lots of reasons why this can happen which depend on all sorts of things like your Sims preferred mode of transport, whether they are getting on or off the subway, if they are meeting up or traveling with other Sims, even down to the orientation and position of the front of lot designation can cause journeys to not work out as you'd expect.

Routing crapness - Sims don't take the most direct and logical route from one place to another.

NPCs can also get stuck in subways and elevators (which is why we list those in the world information) With the Pets EP Sims are now getting stuck on bridges for no apparent reason.

Routing failures - Sims get stuck in particular corners of the landscape, sometimes NPC's can collect in isolated spots which is a significant cause of game lag (there are others).There are many aspects where issues can develop with custom worlds such as: The following additional, specific notes are available for online worlds:

Replacing a World that has been Updated.Can't add new lot spaces in empty world space.This topic contains a few details on some of the issues that players might experience with custom worlds, as well as what we have done to try to address such issues. That is all in addition to coming up with a design and doing what is typically months of building for anything other than a tiny world. To the casual observer it probably seems much like building a lot but on a much bigger scale, but there are a huge number of additional considerations, such as the effect on game performance caused by spawners, effects, terrain mesh density, how close together individual lots are, quantity of textures in addition to things like determining Sim and camera routing, organizing the world layers so that flora can be rendered as efficiently as possible, conforming lot edges to get rid of unsightly gaps, considerations about the transport network, the balance of population verses the number of available jobs etc etc.

Custom sims 3 world with lots of lots