If you’re working in the latest versions of Chrome (~40+) – and maybe Firefox – and you use SPServices, you may start to see an warning:
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience. For more help, check http://xhr.spec.whatwg.org/.
Vigilant SPServices user frankhale reported this to me the other day in the SPServices discussions on Codeplex, which is – at least at the moment – the best place to get help with SPServices. You can also add an issue on Github (sympmarc / SPServices) if that’s your fancy.
The warning is thrown in jQuery, not SPServices, but it’s an SPServices issue.
First off, it’s a warning, not an error. Your code will continue to work in the near term, at least.
So, the bottom line is: “Carry on.” I’ll get a fix into the next release of SPServices for this. In the meantime you should be fine.