Oh this is an easy one.
It was in a game of left 4 dead 2, I believe. Playing No Mercy, and it was the sewer level. At the time (and I still am), I was playing on my exceptionally powerful gaming desktop. Now, I moved last summer, and the Ethernet cables have yet to be set up, so I was (and currently am) using a wireless card for all my internet needs. What's the problem with wireless? Microwaves. Keep in mind that I'm one of those players of l4d2 that's pretty good, and is quick to react and save others. Anyway, it was near the start of the level and we ran to the garage to get the guns. When we got those, we went through the alley to get to the restaurant kitchen, and then all of a sudden, I'm losing connection, but not entirely. I kinda regain connection, but it's really bad. Anyway, my ping went through the fucking roof. When it started, we had just gotten into the kitchen. Turns out, someone downstairs was using the microwave, BIG WHOOP. I quickly ran downstairs, turned the damn thing off, quickly explained the situation, and ran back upstairs. By the time the connection stabilized itself, 1 person died, another was incapped, a 3rd was caught by a smoker on the roof of the safehouse building, and then there was me, who had just a single common hitting me, still with green health, safely in the kitchen. Had the microwave never fucking my connection over, we would have been just fine >_>
TL

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I fucking hate microwaves when I use wireless to play online