It won't necessarily crash. But wrap-arounds, if any, will be off. Type will run off the page on to another, or will draw up short, and type from the next page will end up on the one before. Or hyphens will get screwed. Then you'll get rivers where you painstakingly, and manually eliminated them earlier.
Or the weight won't be right, or the character won't look as nice.
On multiple columns, where you aligned the type so the weight of each side was balanced, is now giving one a much blacker look than another so that it looks as though the page is falling over to one side.
There are so many problems that can come from not using well declared fonts.