I would like to add, as this is the first helpful article in the google results (hello future readers, read xkcd) that some page load actions happen twice, especially where PDF rendering a page is concerned, so you will need to check if you’ve done your action before.

This was me last night

I’ve been struggling this one for a while and appreciate someone else’s frustration with it. But you left out the “best” part (unless I missed the mention) – You cannot trap this exception at runtime. Which makes writing your code to take an alternate path challenging. We have created an ExceptionHandling.cls that logs unexpected errors to a custom object in order to notify a developer and for proactive defect tracking and resolution purposes. So we get this “DML currently not allowed” error at runtime because of the exception logging.

I logged a case with Premier Tech Support on this issue hoping that it might become trappable in the future or to provide a list of areas that this exception may occur so we can avoid them.

Thanks,
Mike