Detailed explanation of the pitfalls of mixing MySQL order by and limit

SELECT * FROM purchaseinfo ORDER BY actiontime,id LIMIT 0,2;

/*
Navicat MySQL Data Transfer

Source Server: Local mysql8.0
Source Server Version : 80018
Source Host : localhost:3308
Source Database : baihe

Target Server Type : MYSQL
Target Server Version : 80018
File Encoding: 65001

Date: 2020-06-09 14:47:37
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for purchaseinfo
-- ----------------------------
DROP TABLE IF EXISTS `purchaseinfo`;
CREATE TABLE `purchaseinfo` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `userId` int(11) DEFAULT '0',
  `inout` varchar(1) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
  `money` int(11) DEFAULT NULL,
  `actiontime` datetime DEFAULT NULL COMMENT 'jiaoyi',
  PRIMARY KEY (`id`),
  KEY `UserId` (`userId`)
) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

-- ----------------------------
-- Records of purchaseinfo
-- ----------------------------
INSERT INTO `purchaseinfo` VALUES ('7', '1', 'I', '10000', '2020-06-09 18:05:41');
INSERT INTO `purchaseinfo` VALUES ('8', '7788', 'O', '20000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('9', '7788', 'I', '20000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('10', '7788', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('11', '1', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('12', '1', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('13', '1', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('14', '1', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('15', '1', 'O', '50000', '2020-06-09 18:05:15');
INSERT INTO `purchaseinfo` VALUES ('16', '1', 'O', '50000', '2020-06-09 18:05:15');