Getting "Internal error: Too many pairs created" without any apparent reason

Hi everybody !

I've been working on a project for two years and the physics part has always been working perfectly.

However, since a week, I started having an error after playing 2-5 minutes : the message I get in the console is "Internal error: Too many pairs created, new pairs will be ignored. This is perhaps due to an excessive number of objects created at one location." Then, it starts ignoring some collisions, but not all.

When I check in the profiler, the number of collision is still the same (18.8k, which is normal I think). I can't understand why I start getting this error. I didn't backup the project so I can't go back.

Has someone already had this error ? What can be the cause if the number of objects or contacts doesn't increase ?

PS : I'm switching between Unity 4.6.4 and Unity 4.6.5 versions.