A mistake that was kind of a technicality. Saying Hitler didn't even stoop so low as Syrian President Assad in using chemical weapons when, in fact, Hitler did use chemical weapons in the concentration camps. I've never been a Trump supporter so it's funny to see his staff stumble, but I do kind of get the valid point that Spicer was trying to make. What Assad has done puts him into the category of Hitler.
As for Holocaust denial, it wasn't that long ago that former Iranian President Mahmoud Ahmadinejad called the Nazi Holocaust a "myth" of European Jewry. Things can be like Hitler in today's world; especially in the Middle East.
Part of my own tribe of "liberals" is making a big deal out of this verbal slip up, which is what journalists and Twitter folks like to do, but in the long run, it may not be that substantive an issue.
A more substantive issue is something like the huge cuts to Amtrak train service that are proposed in Trump's budget request. Amtrak may be taken away from over 200 communities; especially rural communities.