Band members falling out- this could cause one of the band members to not turn up to the gig or even leave the band all together.
Miscommunication- wrong choices could be made and band members could be left feeling like they are not involved.
Forgetting equipment- this could possibly ruin the whole gig if you forget something essential or if the equipment you have doesn’t work and you did’t bring spares.
The songs aren’t ready for the gig- if newer songs haven’t been practised enough so that it is performed at a good quality it could sound bad when performed at the gig and could give the owner of the venue no choice but to not let you perform there again.
You might not sound original or unique- this is important as to be a successful musician you want to stand out to be recognised and remembered.