One of the more relentless ticklers I've worked with in an office setting was named Jennifer.  We'd worked together for about a year and I'd inferred that she was something of a tickler--I'd seen her tickle the knees of another coworker, Amy, and delight in Amy's embarrassed twitches.
But one day I'd sat down at the desk of yet another coworker, Kyla, and--as was my wont--began fooling around with her email as she looked on.  I began composing an email to our boss with a message of something like "I quit!"  Y'know, teasing Kyla with the threat of hitting "send."  Kyla, gratifyingly, freaked out and seized my shoulders and tried to pry me away from her desk, but to no avail.  She laughed and shrieked nervously and said "Someone help!"  People began to gather around Kyla's cubicle.
Then I felt fingers tentatively grip my rib cage and my arms, of course, instinctively shot to my sides, freeing the keyboard for a second.  But then the fingers retreated and so I went back to preventing Kyla from canceling the message.
Then I heard the voice behind me: "Wade's ticklish!"  It was Jennifer's voice.  jennifer's the one who just tickled me.  This, I knew, didn't bode well.
Jennifer resumed tickling me, but with unerring instincts she went this time not for my ribs but for my sides between my ribs and waist, a spot which always throws me into fits and which I couldn't defend with my arms.  Her fingers danced playfully but insistently on those spots, not stopping this time; again I clamped my arms to my sides but this brought no relief.  Kyla set about canceling the message from her email while I sat trapped in her chair--the desk in front of me, people behind me--laughing uncontrollably.  Jennifer just.  Kept.  Tickling.  I thrashed back and forth in the chair but that was about all I could do; no one but Jennifer was going to decide when this stopped.
Finally--finally--her fingers receded again.  I opened my eyes and looked around; the crowd of onlooking coworkers had grown somewhat.  A colleague named Luanne from an adjoining desk said, delightedly, "I heard her go 'Wade's ticklish!'  Then I heard Wade laughing!!"
A cheerful supervisor, Marsha, called from across the cubicle wasteland, "Wade, are you behaving yourself?"
Everyone dispersed, and I presume most people forgot about the incident as soon as it was over.  But Jennifer didn't.  Her favorite refrain over the next two years that we continued to work together was "I know just how to make you twitch!"  And her favorite hobby was doing just that, and only to me (though I couldn't possibly have been the only ticklish person in the office, surely).  Our last interaction before she left for her new job, in fact, was a merciless valedictory tickling that she inflicted in front of many public onlookers as I struggled to maintain composure: "This is the last time I'll get to do this," she grinned.  God knows if she found any suitably ticklish coworkers at her next job.