We were both going upwind and they were on starboard tack (wind from the right) and we were on port tack and therefore had to keep clear. We weren't going to be able to cross in front of them, so the plan was to go behind. Nothing easier than that, just fall off a bit and duck their transom. Something we do dozens of times an evening on a crowded lake.
Well, things didn't work out as planned. A big gust hit, accelerating us towards the mid section of the bigger boat, no matter how hard we yanked at the tiller. Letting fly the sails did not help!
In this instance, although I hate to admit it 
, it was the skipper of the other boat who saved the situation. We simply could not turn downwind.After we recovered from the shock, we looked for a cause and soon, enough, found that the main sheet had twisted around itself in the part where it goes from the end of the boom to the block on the bridle.
This is an older picture (I since have replaced both mainsheet and traveler) but it shows the location I mean. In the picture, the two parts are shown basically parallel, with just a small distance between the blocks. (In the old setup, the bottom block does not rotate, therefore forcing the quarter turn you see in the photo.)
What we saw that evening, was that the two parts had twisted fully around each other 4-6 times, and prevented the blocks from coming as close to each other as would be normal for upwind sailing.
Worse, though, the twists induced so much friction that the boom would not move out, even with the sheet fully released. Normally, the more force you have on the sail, the easier the main sheet releases, here it was precisely the extra tension during the gust that fully locked the twists, whereas earlier in the evening we had been able to let out the sheet as needed, say when going downwind.
The block at the bottom is free to spin, so normally any twist should pull out as tension is put on the sheet. Not this time. After we manually untwisted the sheet by rotating the block, the twist came back each time we tacked. At the dock we found that that end of the sheet had a huge amount of internal twist in the line; unusual, because it's a braided line that should normally not have it's own twist.
Anyway, this is the first time this ever happened to me and, of course, I'm curious to know whether anyone has a suggestion as to a possible cause. (We leave the mainsheet rigged, but take the boom off the mast when not sailing.)
One thing we realized later is that letting fly the jib, even though it helped depower the boat a bit, had the unintended consequence of making it even harder to steer down. But then we didn't realize that the main had been "stuck" until the incident was well past.