What does "forked" mean and how did it make all of my PANDA disappear?

[GAML]

I woke up this morning and all my Pandacoins had disappeared from the mining pool and not AP transactions.  My friend had the same issue with 10 million of them.  He spoke to the admins on IRC and they said that the pool had "forked from the start" and they coins they thought they had were not really there.

Can anyone explain what this means, how it happens and how it can be avoided in the future.  Many thanks.  I am upset that my coins vaporized into thin air =(

[bluemist]

It means that the blockchain for that coin has split into two - it's quite common with alts due to 51% attacks. The wallet for the pool you mined at was connected to the incorrect version of the blockchain, meaning that all coins mined there would not show up as transactions on the correct blockchain. This makes them worthless as they never really existed.

The only way to avoid it really is to mine at a well maintained pool (the pool owner needs to keep the wallet updated and on the correct chain). You can check that a pool is on the correct blockchain by checking that the pool's block height value is correct. coinchoose.com and coinwarz.com generally are a good place to check for the correct block height (but be aware that they sometimes get it wrong), or you could look on a block explorer.

[GAML]

It means that the blockchain for that coin has split into two - it's quite common with alts due to 51% attacks. The wallet for the pool you mined at was connected to the incorrect version of the blockchain, meaning that all coins mined there would not show up as transactions on the correct blockchain. This makes them worthless as they never really existed.

The only way to avoid it really is to mine at a well maintained pool (the pool owner needs to keep the wallet updated and on the correct chain). You can check that a pool is on the correct blockchain by checking that the pool's block height value is correct. coinchoose.com and coinwarz.com generally are a good place to check for the correct block height (but be aware that they sometimes get it wrong), or you could look on a block explorer.

Many thanks.  That is really valuable information.

[InsanityDev]

To expand.

Blockchains are a chain of mined blocks (unsurprisingly). "Longest" doesn't refer to the "height" of the chain, rather it refers to the difficulty of the chain.. so think of "longest" as "most difficult". 51% means that a collection of miners have more than 51% of the computing power of the network, allowing them to create a more computationally difficult chain of blocks.

This chain of blocks becomes the winning chain. The block they start mining from is the "fork" point, and if they succeed then their chain (with all coins and transactions) becomes the one that is classed as "valid", rendering the other should-be-valid chain as redundant.

Checkpoints are pretty important to be in a daemon/qt as it ensures that only the chain with those checkpointed blocks in is classed as valid, this prevents a fork at any point earlier than the last checkpoint block.

It's thus important to:
 - have coin developers that keep checkpoints up to date in clients
 - keep software up to date
 - check the pools keep their software up to date