I get this every so often, and I live in a different country, and it happens on all kinds of phones. My belief has always been that it is a combination of the software not breaking up a message that is too long well, and the fact that there is sometimes unpredictability in SMS. Messages don't always arrive instantaneously, so 2 messages sent in quick succession may not always arrive in the order they were intended, since one might get processed faster than the other.
I personally hate it when people type a 400 character message at once and depend on the phone to break it up. Somteimes it. just. doesn't. work. From one friend in paticular she'll send something that needs to be split into 3 SMS' and sometimes I just won't get one at all. Makes it fun when I miss the middle part of a message.
tl;dr: probably nothing you can do, and tell people to stop trying it.