Correct me if I'm wrong, but forks are only possible with a certain amount of hash power, and to gain enough hash to fork the Bitcoin chain would be very expensive and thus impractical.
yeah you're wrong.
Forks happen anytime 2 miners solve a block referencing the same previous block.
But usually one of the forks gets chosen by consensus right? And after 6 of so confirmations it is extremely unlikely to change.
When this thread was opened there was a bug (or bug fixed) with the latest version of the software which means that the new version of the software chose a different branch to the old, which is a very different and horrible problem. IIRC the developers told people to downgrade to the old version and the associated fork became the chosen fork, and thus losing transactions made in the unchosen fork.
Hammer me if this isn't 100% accurate but I think it is the gist.
Usually forks are OK and get resolved. This thread is about a different 'hard' fork cause by software issues.