The now (in)famous excel 2007 multiplication bug is quiet interesting. First people thought it is a floating point bug but it seems it may not be the case. I did some analysis and found that the bug can be reproduced like this. Take two numbers who when multiplied give 65535(2^16-1). Suppose one of them is A and the other one is B such that A*B=65535. Now if A is a number such that A*10 is an integer and (B*10^N is and integer and B*10^N is of the form 5M (divisible by 5) (where M and N are interger) then this bug can be reproduced.
For instance 10.2(A) * 6425(B)