I have been able to successfully import several CSV files into the Leads table. But a couple times I have got this error:

Fatal error: Allowed memory size of 12582912 bytes exhausted (tried to allocate 129 bytes) in /var/www/html/crm/modules/Import/ImportStep4.php on line 274

I've gone to this file (ImportStep4.php) on line 274 to try and figure out what is going on, but I can't distinguish it.
Can anyone help me with what is going on, and what I need to do to avoid this error in the future?

Thanks.