Minor but at times extremely annoying:
Web forms that, after you have filled in all fields - some via a form-filling utility, like LastPass or RoboForm, but some that you must enter manually like your chosen username and password and other one-time entries - they return the form page as "invalid" because they don't like your email address or they want the 9 digit ZIP instead of 5 or vice versa, or the captcha was incorrect - and it wipes out ALL the custom fields, requiring you to enter the stuff all over again. Sometimes they do not specify clearly enough exactly what they didn't like and force you to refill the form several times before you finally realize what they want.
I rarely ever get to the end anymore, 'cause I usually just mutter a few choice unkind words as I close that tab after the second return! Grrrrr....
BTW - I posted this because it just happened w/Share*It trying to purchase Archivarius. I was using the PayPal option - which though Share*It offers they do all they can to discourage you from using it - and I used the email address that is associated with my PayPal account. Share*It returns it as invalid saying I used a free email address and they don't accept them. First, it is NOT a free email address! Five year subscription cost me a couple of hundred bucks! Plus they want me to use my PayPal address and that is it. Share*It was already on my sh-tlist for other reasons, like how they screw people with undisclosed exchange rate fees and other added little annoyances. This one was the last straw though.
I won't be purchasing Archivarius.