How to fix "Incorrect string value" errors?

MySQL’s utf-8 types are not actually proper utf-8 – it only uses up to three bytes per character and supports only the Basic Multilingual Plane (i.e. no Emoji, no astral plane, etc.).

If you need to store values from higher Unicode planes, you need the utf8mb4 encodings.

UPDATE to the below answer:

The time the question was asked, "UTF8" in MySQL meant utf8mb3. In the meantime, utf8mb4 was added, but to my knowledge MySQLs "UTF8" was not switched to mean utf8mb4.

That means, you'd need to specifically put "utf8mb4", if you mean it (and you should use utf8mb4)

I'll keep this here instead of just editing the answer, to make clear there is still a difference when saying "UTF8"

Original

I would not suggest Richies answer, because you are screwing up the data inside the database. You would not fix your problem but try to "hide" it and not being able to perform essential database operations with the crapped data.

If you encounter this error either the data you are sending is not UTF-8 encoded, or your connection is not UTF-8. First, verify, that the data source (a file, ...) really is UTF-8.

Then, check your database connection, you should do this after connecting:

SET NAMES 'utf8mb4';
SET CHARACTER SET utf8mb4;

Next, verify that the tables where the data is stored have the utf8mb4 character set:

SELECT
  `tables`.`TABLE_NAME`,
  `collations`.`character_set_name`
FROM
  `information_schema`.`TABLES` AS `tables`,
  `information_schema`.`COLLATION_CHARACTER_SET_APPLICABILITY` AS `collations`
WHERE
  `tables`.`table_schema` = DATABASE()
  AND `collations`.`collation_name` = `tables`.`table_collation`
;

Last, check your database settings:

mysql> show variables like '%colla%';
mysql> show variables like '%charac%';

If source, transport and destination are utf8mb4, your problem is gone;)

The table and fields have the wrong encoding; however, you can convert them to UTF-8.

ALTER TABLE logtest CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci;

ALTER TABLE logtest DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;

ALTER TABLE logtest CHANGE title title VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_general_ci;