Using the following /data merge command on any sign entity results in the console log "[Server thread/ERROR]: Input is not a list of 4 elements". The data merge is however executed successfully, with the back text of the sign being missing but the front text being visible. Notice that there are 3 back text messages and 4 front text messages.

/data merge block ~ ~ ~ {back_text:{messages:['{"text":"1"}','{"text":"2"}','{"text":"3"}']},front_text:{messages:['{"text":"1"}','{"text":"2"}','{"text":"3"}','{"text":"4"}']}}

The merge wasn't completed successfully, so the command should return false. A repeating command block will spam this message to the console every tick that the merge command is executed.