The poly morphed method handleIfUnmodifiedSince(final HttpServerExchange exchange, final Date lastModified) at https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/util/DateUtils.java#L207 calls handleIfModifiedSince instead of handleIfUnmodifiedSince(final String modifiedSince, final Date lastModified) at https://github.com/undertow-io/undertow/blob/master/core/src/main/java/io/undertow/util/DateUtils.java#L218

Problem exists since 1.1.x branch till latest master.