Are you using a spike or Spanish point razor? This has happened to me with a spike point before and I know it was not my technique. Some where in shaving I may have bent the blade, but after stropping (and it did scratch the strop like the pictures here) but it shaved great after. May have just been my razor though.

The big thing is just practice. Pull the strop tight, and then relax your muscles keeping the strop straight thus making it taught. In videos you often see a curve in the strop, you have to realize the video is at a side angle and you are looking from above. You shouldn't see the bend at all.

Last when you strop the razor just set the razor on the strop. Don't put it on. Then only use enough pressure to keep the blade and spine on the strop while moving it back and forth. Strop slow and stop about every 10 laps and think about the pressure you are putting on the strop. If you have increased in pressure let up, if you are good then keep going.

Keep at it and you will gain the proper technique. Just remember taught and light pressure and you will be just fine.'