Only one surprise "tough" card for me, since I don't pursue any set all that agressively. Somehow in 30 years I just didn't end up with a SC 150 factory overprint. Not that it was all that hard to fix once I realized I didn't have one.
The toughest card to actually get has been an 88 topps Canseco. Last one I needed, pre-Ebay and I didn't want to buy a whole set. 2 years of asking around, people had them, but didn't want to bother digging in the back room for a 10 cent card.
Steve B