1) First night of playing games with my Audio Technica ATH-AD2000. I only used headphones 25% of the time with games, but because of these I now never go without using a headphone. Right now it's a mix between the HD-598 and ATH-AD2000. ATH-AD2000 and the DJ100 are still the only headphones that have "Wowed" me. A few have come close. I guess I'm not easily impressed and usually too picky. I used to think that headphones past say $300 are only going to be "slightly" better, but the ATH-AD2000 has definitely proven me wrong. Unlike most here, I don't think I could ever justify the purchase of an $800+ headphone. I guess I don't like music enough to do that. Maybe if I had listened to music for hours and hours each day or had thousands of albums. If I actually hear a $800+ headphone I may think differently. Who knows.
2) Probably the first week of hearing the DJ100. I just couldn't believe how good they sounded for $80. My near perfect sound signature. Everything sounded crystal clear and natural. As if that was how everything in "real life" would sound. I find it quite funny how an unknown $80 headphone can be far better for my ears then anything else under $150 (or maybe even $200!). It's the one headphone that makes music just fun to listen to. Kind of makes me wonder why I spend so much more money on other headphones that sound half as good. Sorry, went into fanboy mode there.
My worst yet.....
Trying to replace an old Koss headphone's cable. I didn't know how to solder at the time so it was all a learning experience. I spent hours trying to get the silver cable working only to find out the driver was blown. All that money wasted. What made it worse is that the wiring inside was so confusing because of a stereo/mono switch. Then another problem was attaching a plug with super thin silver cable that seemed to break. If that wasn't bad enough the plug eventually just fell apart. Oh and to make everything twice as bad, the soldering iron would only work 25% of the time. Some defective battery powered iron. Those things are a joke. Two defective Radioshack items in one night! Never had something like this happen before!
So now I had some silver wire that I had wasted money on, so what did I do? Attached it to a KSC75 for my own amusement! It was stupid easy luckily, but it ruined them. Sounded terrible, but did add a ton more detail. Un-sleeved silver cable for a headphone is the most annoying wire in the world. Sounds and feels like thin metal coat hangers when you move them.
Another...
Accidentally blowing up my Total Airhead amp because of plugging in the wrong AC Adapter and smelling fried electronic parts. Oops! $100 down the drain. I ended up buying the same one and actually LABELED the adapter this time.