From time to time BBC BASIC (Z80) checks to see that the program in memory is of the correct format. If it is unable to follow the program from the start to the 'program end marker' it will report this untrappable error. The error can be caused by a read error, by only loading part of the program or by overwriting part of the program in some way. (Machine code programmers beware). Without a full understanding of how a program is stored in memory, there is little you can do to recover a bad program.