Ah yes. The first of many double length specials.
There's a couple noticeable things here so let's get started.
First, there is a small layering error on Phineas' mouth when they first go back in time. In order to save having to draw the character over and over again with different lip flaps, animators will usually draw the character with a static pose and layer a separate lip flap over top of it. This is very common in productions all over the world. But occasionally, layers may be turned off for a frame leading to Phineas becoming Rayman for a frame.
Second, this episode is Carl's first on screen appearance! Funnily enough, his first appearance comes with a flub. His outlines are not colored in properly :P
Finally, the name of Perry's agency, now known as the Organization Without a Cool Acronym (OWCA) was originally named Super Secret Spy Network (SSSN). The name OWCA wouldn't be introduced until the season 2 special "Summer Belongs to You!" and the SSSN name was used very sparingly in the first season, the only other time it's seen is in "Phineas and Ferb Get Busted!" However, the SSSN name and logo were used on various early Phineas and Ferb merchandising, like the one pictured above