Warning: array_flip(): Can only flip STRING and INTEGER values! in DrupalDefaultEntityController->cacheGet()
This is related mostly to the issue in your code or bug in the module which you're using.

Basically you should validate your DrupalEntityControllerInterface implementation, and you should not patch your Drupal core as suggested by @vgoradiya since it's not a bug in Drupal core.


  Consensus seems to be that issues arise with bad data and should be corrected at the offending module level, not by adding a safeguard to DrupalDefaultEntityController that would mask deeper problems. Having an error here is a warning to developers that they are using the D7 APIs incorrectly. - @jessebeach


So the challenge should be to identify which module implements Entity API incorrectly.

See also:


array_flip() [function.array-flip] issue in DrupalDefaultEntityController / entity.inc which is not a bug, but closed as 'works as designed',
Entity API Tutorial,
How to create a new entity type
Entity API at Drupal Groups
Creating your own entities with Entity API




If you're developer of the module which causing the issue, please check the followings:


Make sure your array won't contain NULL values, otherwise use the following trick:

$arr = drupal_map_assoc($arr, function($v) { return (string) $v; });

Check the Examples module (entity_example) for Developers how to implement entities.
You're loading entities using the right $ids structure.
Use debugger or debug_backtrace() to debug the issue.