I just used the improve option on a suggested edit. Specifically, this one, if it matters.
It was not very helpful (it just added an unnecessary platform tag), so I unchecked the box, but every time I tried to submit it, I'd get a 500 server error.
I tried a few different things (leaving the tags alone, leaving the body alone, typing in a revision summary), but the culprit seems to have been the "was this helpful" checkbox. As soon as I left it checked, the suggested edit went through fine.