how is that even possible?
What is wrong with the other TX that don't get confirmed (over 50 hours old)?
Transactions can stay in the mempool for 7 days if they are found in an orphaned block:
#define CRYPTONOTE_MEMPOOL_TX_LIVETIME (86400*3) //seconds, three days
#define CRYPTONOTE_MEMPOOL_TX_FROM_ALT_BLOCK_LIVETIME 604800 //seconds, one week
Are transactions not validated before they enter the mempool?
Well a tx is broadcasted into the mempool and nodes validate it. The older transactions you reference are almost certainly in orphaned blocks and will drop out of the mempool when they are older than 7 days. If it failed any other verifications (such as double spend or inputs not fully spent) it would drop out faster.