I had a customer for years, was on retainer for a few thousand dollars a month. I knew their systems inside and out, managed their hosting infrastructure. They couldn't hire anyone to take over -- not a glamorous business, didn't offer competitive salary, not a great workplace (I worked remote). I reported to the COO and all was good, I would deal with whatever problems came up and implement whatever upgrades or enhancements they needed.

The COO left and the CEO/founder took over. I didn't have much day-to-day interaction with him before that. He called a meeting and told me I had to start submitting detailed invoices and charging hourly. I said no, I preferred the retainer arrangement and they got guaranteed priority response from me in return, and no unpredictable monthly bills. He then asked "What exactly are we paying you for?" I said they were paying me not to quit. He didn't exactly like hearing that, but after he talked to his managers he dropped the issue, and I kept the gig on retainer for two more years, until I fired them for asking me to do something legally questionable and definitely unethical.