I think "always use getLastError" is a poor general guideline. The "gotcha" at the heart of it is to know that mongodb syncs inserts to disk at a configurable interval (1 minute by default) - which means data can be lost. Calling getLastError (or setting safe => true in a lot of drivers) will force the write to disk.

However, one of the great things about MongoDB is that, in some cases, you can easily afford to lose 1 minute worth of inserts in exchange for huge performance gains. Large chunks of data that is used for analytics is a good example since lost data won't [likely] impact final aggregates/percentages.

Some of our inserts, like user registration we run with safe=>true. Others, like audit logs (which, for us aren't as important as they might be for others) we don't.