I stumbled across this at work the yesterday and I'm still not sure what it should be. This is how I found it written by our Britain-born client:
If your e-mail address or mobile number have changed since the last time, please let us know.
Does that sound right? Or more important, is it correct even if it doesn't sound right? How about:
If your e-mail address or mobile number has changed since the last time, please let us know.
It could be that both e-mail address and mobile number are now different. Therefore both of these things have changed. However, it's unlikely in general that either change. It's then also extremely unlikely that both have changed. In most cases, if there is a change, there is only one change.
Neither have or has seem to fit perfectly and I'm making a big deal out of this. It smells like the best solution is to rewrite it like this which circumvents the problems nicely.
If your e-mail address, mobile number or both change changed since the last time, please let us know.
English experts, thoughts?
PS. Computer geeks, resist the temptation to suggest the use of xor instead of or to solve the problem.