The camera was a Sony A7 III and it had been my main body for two years. It had never given me a problem. I had done about ninety paid shoots with it. The bride was walking down the aisle when the screen flashed an error code and the camera shut off. I stood there for two long seconds with a dead camera in my hands while the most important moment of someone's day was happening twelve feet away. The second shooter caught it. I did not. That night I drove home and could not sleep, and the next morning I made a list of every single piece of gear I needed to bring to a shoot, and I doubled it.
Equipment redundancy is the difference between a hobbyist and a professional. The hobbyist brings one camera and hopes it works. The professional brings two cameras and assumes one will fail at the worst possible moment. The same logic applies to every other piece of gear. Lenses, batteries, memory cards, audio recorders, lights, tripods, cables. If the shoot will be hurt by the failure of any item, you bring a second one. The cost of the backup gear is always less than the cost of failing a paid client.
For cameras, the simplest version of redundancy is a primary body and a secondary body. The secondary does not have to match the primary, but the file format and color science should be close enough to grade together in post. I run a Canon R5 C as my primary and a Canon R6 Mark II as my secondary. Both bodies record similar codecs and both can be color-matched in DaVinci Resolve in about twenty minutes. The secondary body lives in my car for solo shoots and in my second shooter's hands when I have a team. If my primary dies, I am operating again within sixty seconds.
Batteries are the cheapest insurance you can buy. A Canon LP-E6NH battery is $89. I carry six on a small shoot and twelve on a wedding. The math is brutal in the other direction. If your one battery dies in the middle of a vow exchange, you are not getting that footage. Ever. Six batteries cost $534 once and last for years. Six minutes of missed footage on a wedding can cost you $4,500 in refunds and rebuilds and reputation damage. The backup gear is not an expense. It is risk management.
Memory cards follow the same logic. I run two cards in every camera that supports dual slots. The R5 C has a CFexpress slot and an SD slot, and I record to both simultaneously. If one card fails, the other has the same footage. CFexpress cards are not cheap. A 256GB ProGrade card is around $300. The SD card serves as a real-time backup that costs about $40. The cost difference between recording to one card versus two is roughly the price of a nice dinner for the entire job, and the protection is total.
Audio is where most videographers fail to build redundancy because audio gear is unfamiliar territory. I run two systems on every shoot. A Rode Wireless Go II handles the lavalier on the subject. A Zoom F3 with a shotgun on a boom or a stand records the same audio path as a backup. If the lav fails, drops out, or has rustle, the boom recording is there. The Zoom F3 records 32-bit float, which means I cannot clip it no matter how loud the source gets. The total cost of this setup is around $750. The cost of unusable audio on a paid shoot is the entire shoot.
Tripods and supports are easier to overlook. I always have two. A Manfrotto 504X for the primary camera and a Sirui SH-25 for the secondary. If one head fails or a leg locks up, I have a working option in the trunk. I have only had a tripod fail once in nine years. Once was enough.
Lighting redundancy depends on whether you are doing controlled or run-and-gun work. For controlled lighting, I bring more units than the scene calls for. If the lighting plan needs three Aputure 60D lights, I bring four. The fourth is in the case. If a light fails to turn on, I am pulling the spare in under two minutes. For run-and-gun, I rely on natural and ambient light primarily and bring a single small panel as a fill. The panel has a backup battery. The battery has a backup battery.
Cables are the smallest line item and the most likely point of failure. I keep a small bag with two of every cable I might need on a shoot. USB-C, mini-HDMI to HDMI, XLR, 3.5mm to XLR, AC power cords, BNC for timecode. The whole bag costs less than $200 and it has saved me probably ten times in the last two years.
The mindset that supports all of this is a question I ask before every shoot. What happens if this single piece of gear fails right now, in the middle of the most important moment of the day? If the answer is that the shoot is ruined, I bring a backup. If the answer is that I can work around it, I do not. The cost of carrying extra gear is annoyance and weight. The cost of failing a paid shoot is your career.