In "Girl Trouble", Helga manages to be both in her seat and standing at the front of the class sharpening her pencil. At the same time, all the desks appear to be arranged in rows, but when she talks to Arnold seconds later, they are in groups.
In "24 Hours to Live", Helga calls and says "22 hours and 46 minutes until you die,". He then talks to Grandpa and goes up the stairs and talks to Grandma. The phone rings saying Arnold has 22 hours and 14 minutes. This means that it took half an hour to walk up the stairs.
In "Biosquare", Arnold and Helga enter the greenhouse at 8:00 AM and must stay in there for 24 hours. When they wake up the next morning, Arnold claims there is six hours to go. This means it is 2:00 AM yet the sun is up and shining.