A long SMS is just a series of normal SMSes in disguise. Some of the SMS contents is reused as an indicator saying, "this is part 1 of 3", "this is part 2 of 3", etc. Apparently you only got the first part and your phone gave up waiting for the rest. Does it happen every time or just occasionally? Perhaps the sender has run out of credit and only the first part has actually been sent in the first place?

EDIT: Just realized this is on Nitroid. Perhaps Nitroid has a genuine issue with long SMS, I have no idea.