With regards to references... it depends on where you are because current legislation in UK states that you are not allowed to give a bad reference of any form. If you hated the employee, he burnt your house, stole your car and pissed on your desk in a fit of spite the only thing you are allowed to say in a reference is 'this person worked for our company between x date and y date'

Of course, most HR departments know the code and know that this statement on its own with no other statements or points made means 'do not hire' and there is nothing to stop you talking 'off the record' to other employers in the area at various functions you all attend...

I do agree with Delia that any activity that looks like it may be sexual harrassment or discrimination (even something like 'make me a coffee Miss Jones' is suspect in this day and age) will get take seriously even if both parties claim it is 'a bit of fun'. The trick, like with the law and accusations of GBH etc, is to not draw attention to anything unusual. In most cases, the only thing that will cause trouble at high level to the extent that HR are aware of it is when one or more people complain about it, especially a participant, but it does not do to make it blatant.