Entering (and losing) programs on HP 48

[calculon]

After years of using HP calculators for simple tasks I've decided to take the plunge and learn how to program the HP 48, starting with the information in the HP 48 Owner's Manual. But I've hit a snag. One program from this book that I've definitely keyed in correctly can't be put on the stack because of an "Invalid Syntax" error. This means losing the work... or does it? I'd be very surprised if the calculator can't save a partially or wrongly entered program but I can't find anything in the Manual or online to enlighten me. (The program that I've now typed in and lost twice is BDISP on p. 31-10. The error comes at the very end, before PRESERVE that was previously entered successfully.)