How often do game developers miss deadlines/milestones due to unexpected things?
For developers in the trenches like me or my team, it's a fairly common occurrence that individual contributors will not be able to finish certain tasks by a given deadline. Sometimes the estimates and the actual work needed are nowhere near congruent, sometimes there are dependencies that took longer to work out than expected, sometimes there are unpredictable events like system outages or developers needing to take a medical leave of absence. In such situations, we adjust what we're delivering for this milestone and that task and any of its dependencies usually gets "punted" to the next milestone. Individual tasks can often shift back and forth without affecting the overall milestone too much.
It is actually possible for an entire milestone to slip as well - if the mission-critical deliverables for the milestone aren't done in time for whatever reason, the milestone isn't going to be accepted by the money people. Remember, the milestone schedule is originally pitched by the studio and agreed to by the publisher. It outlines exactly what the studio needs to deliver and demonstrate at each milestone and when each milestone should be there. Independent developer studios are often paid by the milestone, so missing or delaying a milestone delivery can mean that we don't get paid until we deliver.
When studios start missing milestones, the publisher often steps in to "meddle". Missing an entire milestone by a significant time frame is a big deal and often means the entire delivery schedule needs to be re-negotiated and re-planned. It's a sign of development hell and a project in a lot of trouble. From the publisher perspective, this is saving the project - things are already off the rails and need fixing or the entire project will need to be cancelled and the studio cut loose.
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