lmirante

Actually it is complete opposite - if your flash lasts for time long enough 1/250s is perfectly possible (that's why with continuous light you can use any shutter speed, because it lasts long enough).
 
Let me explain now - when you have SS set to 1/180 or lower, second curtain starts to close after the first curtain is fully opened. So let's say we have very fast flash - sequence looks like this:
 
1. First curtain starts to open
2. First curtain opens fully
3. ZAP - flash just had been triggered and it lasts 1/2000s
4. Second curtain closes
 
With SS set to 1/250 :
1. First curtain starts to open
2. Second curtain starts to close after 1/180
3. First curtain opens fully 4. ZAP - flash fires for 1/2000s (but a small bit of flash is already covered that's why we have thin black bar)
5. Second curtain closes  
If we will set SS to 1/250 and rear curtain flash we will have:
1. First curtain starts to open
2. ZAP - flash fires for 1/2000s (but a small bit of flash is already covered that's why we have thin black bar)
3. Second curtain starts to close after 1/180 4. First curtain opens fully 5. Second curtain closes
  Basically black bar should be at the top or bottom of the frame (in panoramic view) depending on the sync type - whether flash is triggered after first curtain is fully open - scenario 1 - or right before the second starts to close - scenario 2).   Now let's think about hypothetical flash which lasts for 1s:  
With SS set to 1/250 :
1. First curtain starts to open
2. Second curtain starts to close after 1/180
3. First curtain opens fully 4. ZAP - flash starts
5. Second curtain closes
6. Flash stops    We still have black bar   If we will set SS to 1/250 and rear curtain flash we will have:
1. First curtain starts to open
2. ZAP - flash starts
3. Second curtain starts to close after 1/180 4. First curtain opens fully 5. Second curtain closes
6. Flash stops
 
Only in this hypothetical situation we end up with fully exposed frame.
   

You are not missing anything. At 180/1" manufacturer guarantees (or at least has to ) no issue with synchronization. On most cameras it's can be higher where black bar start appearing. As well it depends on the triggering system you use and the chain of them (pocket wizard activates younguo which activates Cactus). My XT1 works 90% at 1/200" with my Canon's 600EX and Profoto's. And sometime the bar is very slim and its easy to crop

Thanks for the entertainment.  Not for nothing, but fstop's suggestion is the most likely to work with my particular husband.  He'd also appreciate the i7 vs. i5 argument.  When building my computer I got an NVIDIA GTX 670.  2 weeks later his video card died and he needed an NVIDIA GTX 680.  Maybe I should do some tasteful almost nudes with an 18mm and the manual 100mm I have and show him the difference.  And tell him if I can have the 90mm, the "almost" goes away.......