Intermetent 500.25 errors

Sometimes my ARR and URL rewrite rules are working great, but sometimes when I send what I think are identical requests, I get an HTTP 500.52 error.

I notice that on the request that fails, it is using the REWRITE_FROM_CACHE_ACTION, but the request URL at the URL_REWRITE_END is the same. Does this make sense to anyone?

Thanks so much for your help!