How would gene expression be affected if there was an error in transcription (writing instructions) ?

Gene expression is where the mRNA is made. If there's an error in transcribing the DNA into mRNA there could be a major effect on consequent gene expression if:

1. the original gene is involved in gene expression itself (e. g. the RNA polymerase) ... if it's not made correctly you would have serious future problems.

2. if the original gene is for a protein involved in further gene expression (e. g. transcription factor/regulator) - if made incorrectly it may not be there to properly control gene expression so you could get other genes expressed when they shouldn't be, or not expressed when they should be.