Flippin' Sheets. Use the tweak tool to flip the section sheet from the top. No relation to Flippin, Arkansas.
In this puzzle, the second sheet from the top has two hydrophobics with blotchy yellow exposeds on them.
In the previous puzzle, the tweak tool was used to rotate a helix. In this puzzle, the tweak tool is used to "flip" a sheet. This turns the sheet over and moves it over a bit. In this case, flipping the sheet will hide the two hyrdophobics in the core of the protein.
Right-click anywhere on the second sheet and select "Tweak" from the wheel menu. The tweak tool shows purple arrows next to the sheet. Click on the right side of one of the arrow to flip the sheet.
Flipping the sheet is the easy part. This puzzle has the reputation of being a little bit "touchy", but "buggy" is probably a better term.
For some reason, the shake tool doesn't produce points after you flip the sheet. Shake should so something in this case, since lots of sidechains have moved. The protein will fly apart if you wiggle it after the flip.
To get around this, freeze all four sheets by shift double-clicking on each one. Then start to wiggle. The protein will still pull apart, but it should start to come together again. When the sheets are bonded again, stop the wiggle and shake. Shake should now gain points. Stop the shake, and click on "Unfreeze Protein" (or keyboard shortcut "f"). Then wiggle again. This should be enough to complete the puzzle.
As usual, you can shake and wiggle a few more times to improve your score.
You can also draw bands between the sheets to hold them together, instead of or in addition to freezing them.
Technical stuff: yes, Foldit has bugs. The Computer Science kind, not the Biology kind. Foldit player LennStar pointed out that this intro puzzle had problems way back in 2009.