It was a denial of service attack, not in the sense of soaking up my brain cells solving puzzles, but in causing me to devise and program my own solver. (In Java, text console only.) Once I wrote a solver, I felt as if I had solved all puzzles.

Then I got interested in devising puzzles with multiple solutions. Not too difficult. But making a few puzzles with two solutions was fun.

Experiment_203(

   " 1 . . | 2 . 8 | . . 9 "+
   " . 8 . | . . . | . 3 . "+
   " . . 7 | . 1 . | 2 . . "+
   //------+-------+--------
   " 4 . . | 1 2 3 | . . 6 "+
   " . . 2 | 4 5 6 | 9 . . "+
   " 6 . . | 7 8 9 | . . 4 "+
   //------+-------+--------
   " . . 6 | . 4 . | 8 . . "+
   " . 2 . | . . . | . 7 . "+
   " 9 . . | 8 . 2 | . . 1 "
  ),

// Solution #1. Found in 0 days 00:00:00.004.

// 245 boards examined so far.

   1 6 5 | 2 3 8 | 7 4 9
   2 8 4 | 6 9 7 | 1 3 5
   3 9 7 | 5 1 4 | 2 6 8
   ------+-------+------
   4 7 9 | 1 2 3 | 5 8 6
   8 3 2 | 4 5 6 | 9 1 7
   6 5 1 | 7 8 9 | 3 2 4
   ------+-------+------
   7 1 6 | 3 4 5 | 8 9 2
   5 2 8 | 9 6 1 | 4 7 3
   9 4 3 | 8 7 2 | 6 5 1

// Solution #2. Found in 0 days 00:00:00.001.

// 287 boards examined so far.

   1 6 5 | 2 3 8 | 7 4 9
   2 8 4 | 9 6 7 | 1 3 5  // <-- 9 6 7 instead of 6 9 7
   3 9 7 | 5 1 4 | 2 6 8
   ------+-------+------
   4 7 9 | 1 2 3 | 5 8 6
   8 3 2 | 4 5 6 | 9 1 7
   6 5 1 | 7 8 9 | 3 2 4
   ------+-------+------
   7 1 6 | 3 4 5 | 8 9 2
   5 2 8 | 6 9 1 | 4 7 3  // <-- 6 9 1 instead of 9 6 1
   9 4 3 | 8 7 2 | 6 5 1


   2 total solutions found.
   304 total boards examined.
   Total time 0 days 00:00:00.041.

Then I got to looking at difficult puzzles on the web. Apparently AI escargot is the world's most difficult. (And the site http://www.aisudoku.com/index_en.html says I can't publish the board). So I'll only publish the stats of applying my solver to it.

  Solution #1.  Found in 0 days 00:00:00.029.
  3,906 boards examined so far.

  1 total solutions found.
  7,832 total boards examined.
  Total time 0 days 00:00:00.085.