Every day, no exceptions

The mistake

Two pricing scripts each poll the other's public price once a minute and write back that value minus ten pounds, so each run feeds the other a lower input. With no floor, no sanity check on the size of the change and no detection that the upstream price is itself derived from your own, the pair forms a closed feedback loop that ratchets down ten pounds a minute until it hits zero and then goes negative. Because the writes go straight to the live storefront, the loop is also taking real orders the whole way down.