It usually means that a Berkeley DB repository has run out of database locks (this does not happen with FSFS repositories). It shouldn't happen in the course of normal operations, but if it does, the solution is to run database recovery as described here. If it happens often, you probably need to raise the default lock parameters (set_lk_max_locks, set_lk_max_lockers, and set_lk_max_objects) in the db/DB_CONFIG file. When changing DB_CONFIG in an existing repository, remember to run recovery afterwards.
If you're using http:// access, "Cannot allocate memory" errors show up in the httpd error log and look something like this:
[Wed Apr 07 04:26:10 2004] [error] [client 212.151.130.227] (20014) Error string not specified yet: Berkeley DB error while opening 'strings' table for filesystem /usr/local/svn/repositories/svn/db: Cannot allocate memory [Wed Apr 07 04:26:10 2004] [error] [client 212.151.130.227] Could not fetch resource information. [500, #0] [Wed Apr 07 04:26:10 2004] [error] [client 212.151.130.227] Could not open the requested SVN filesystem [500, #160029] [Wed Apr 07 04:26:10 2004] [error] [client 212.151.130.227] (17) File exists: Could not open the requested SVN filesystem [500, #160029]