Assuming the execute has been set, you could check whether you not accidently entered some control characters
Code:
$ hexdump -C script.sh
00000000 23 21 2f 62 69 6e 2f 6b 73 68 0a 65 63 68 6f 20 |#!/bin/ksh.echo |
00000010 22 68 65 6c 6c 6f 20 77 6f 72 6c 64 22 0a 0a |"hello world"..|
0000001f