Running out of memory, how can I reduce the size of wallet.dat?
Bitcoind uses a lot of virtual memory, due to having multiple threads, and maintaining several node-wide and per-peer caches of data (UTXO cache, mempool, relay cache, network buffers, known invs list, ...). 

Until recently, many of these were effectively unbounded, depending on network conditions. The upcoming release of Bitcoin Core 0.12 has far better internal memory accounting, UTXO set limiting, mempool limiting, and more compact internal representations.