Over the nearly twenty years, a few things change. Notice that the "taste" line on the side of the mug changes. In 1997 a taste was nearly half a mug, while in 1998 it was 1/3 of a mug. Today a taste is closer to 1/4 of the glass. In 2013 the Festival switched to using glasses, which allows for a cleaner taste of beer.
I noticed that in 1994 the Fest included an apostrophe - "Brewer's Festival," - implying the fest is for brewers, rather than a celebration of brewing. Was this a misprint, or a deliberate act? Was the apostrophe dropped to become more inclusive?
Fun fact: A child's sippy cup lid fits securely on top of the OBF mugs, so you can use them as a no-spill substitute for your child (or yourself). The mugs are lightweight and unbreakable. A lot of mine are beat up because I take them camping. The handle makes them easy to strap onto the outside of a backpack when hiking.
Here are some of the slogans from past years
1993 - Greetings from...(the OBF) Wish you were here
1994 - In Search of the Holy Ale
1995 - Happy Ales to You!
1997 - I'm Hoppy!!! at the Oregon Brewers Festival
1998 - Celebrating Beer
1999 - We've got the Brews
2000 - Jack of all Ales
2002 - Celebrating American Beer
2005 - Lewis & Clark Expedition - Meet us at the end of the trail.
2008 - Celebrating the 21st Amendment
2011 - Celebrating 24 Years of Amercian Craft Beer - 85 Beers
2012 - Cheers to 25 Years
Click on the images to see a larger version.