I joined as their 3rd employee - as a junior developer to assist the current senior. When they saw how good I was, they sacked the senior. I was given a pay cut, ostensibly to keep the lights on, prettied-up with a new contact saying if after a year they were still in business I'd get my salary restored, but a major bonus if they reach profitability or were bought out within a year. Cue 12 months of being an idiot, regularly working weekends and all-nighters to add major features on little notice for meetings with potential customers and developers. New projects were piled on with no regard for workload or realistic deadlines. They needed a new DC, but rather than hire a sysadmin they passed it to me because I'd had experience on my CV.
By this point I'd lost sight of salary and bonus - I was working to try to avoid letting the team down by missing deadlines. As absurd as it sounds now, the atmosphere in the office made me feel I was part of something special, I was doing my part, and I was going to anything I could to keep up my part. I'd been hired part-time, but I worked full time, and then some, spurred on by my mis-placed sense of loyalty and necessity to cope with my workload. Then at the end of the year they said they couldn't afford to give me back the pay cut, and a couple of months later they announced they were going to be acquired - conveniently close to the year end to be a coincidence. I realised that I wasn't part of the team, I was just employee 3. I'd already started to suffer major burn-out / a bit of a breakdown, so I quit. That was years ago and I'm only just starting to get myself back together.
I blame myself entirely - I was naive and let myself be manipulated and used. By that point I was so deep into the "we're all in this together" that I couldn't see what was happening. But you're not in it together - there may be more honorable founders out there, but at the end of the day, you're working to line their pockets, and just because you've got a bit of paper saying they'll be nice doesn't mean they will. Trust no-one, get everything you're owed up front, and remember at all times that for you it's just a job.
Work to live, don't live to work.
(From a throwaway account for obvious reasons. Ahh, that's better ;) )