Amanda Bynes -- It Would Be Funny, If It Weren't So Sad

Your first reaction to watching Amanda Bynes ride a bicycle haphazardly through the streets of Manhattan might be to laugh ... but this is going to end badly unless she gets some help ... STAT.

Bynes was spotted this weekend riding a Citi Bike ... where she nearly took out a few pedestrians as she sloppily rode through a crosswalk. The video was shot right near Penn Station ... one of the more heavily trafficked parts of the city.

Early Monday morning, a Twitter account believed to belong to Bynes posted a photo of a lit cigarette that appears to be weed ... but Bynes loves to say how she doesn't smoke pot. And if you believe that, we have a bridge we'd like to sell you ...

As TMZ first reported, Bynes flew from L.A. to N.Y. on Friday and her behavior was all over the map. Amanda's friends all think Amanda needs help ... or else she might wind up in serious trouble.