I imagine it would be quite hard to come up with good debugging examples for a class. Anything that has one right answer is going to smell like a made-up classroom assignment. The thing that makes debugging hard is multiple hypotheses (or amazingly, exhausting all hypotheses). Your crash can be caused by errors on many different abstraction levels, so your debugging process has to be thought about on many levels. On…
Have bugs with index bugs like:
for(var i = 1; i For SQL use a JOIN where it needs a LEFT JOIN.
I can think of others off the top of my head. Leave out an optional parameter that's been populated from the front-end, but don't pass it to the method call. In OO languages, have a constructor accidentally call itself instead of the alternative with more parameters (infinite loop). Nest two for loops and accidentally use i instead of j inside the inner loop. Have an if statement that looks sorta right but is logically impossible to satisfy.
For the "only sometimes crashes" bugs, div by zero is an easy example.
All the sort of accidents you instinctively correct now are probably good beginner debugger exercises.