If Apple ends up losing their impending court battle with the FBI, and are compelled to help unlock the 'San Bernardino iPhone' through the creation of a custom software update, the engineers set to work on this task may throw up some roadblocks.
According to The New York Times, which talked to "more than a half-dozen current and former Apple employees", engineers tasked with breaking in to the iPhone may refuse to complete the work, while others may quit their jobs entirely. For some, quitting the company is a better option than helping defeat the security measures they designed in the first place.
This may present some issues for the FBI. While a court order can force Apple to help break into the iPhone, there is nothing stopping Apple employees from quitting rather than helping the FBI. If enough engineers decide to leave, it may take a very long time for others to force their way inside the device.
Apple is already on the offensive against the FBI in regards to the San Bernardino iPhone. The company has stated that circumventing the security measures they implemented is completely against their values, and they will fight to prevent the FBI from forcing their way in and creating dangerous precedents in the process.
In the event that the courts decide against Apple, the company believes it would take a team of around six to ten engineers between two and four weeks to develop the update needed to break the iPhone. If any of these engineers decide to resist or quit, that timeframe would extend.
It's looking likely that this case will drag on for years, so engineers at Apple can continue to work as normal for a while yet. But when the time comes, it's not going to be a straightforward task to comply with the FBI's demands.