Orient defender Alan White was sent off in stoppage following a stormy goaless stalemate at Brisbane Road.
The O's were left kicking themselves for failing to make their domination count and then suffered another blow then White became the first player to see red for Orient this season following a clash with Boston substitute Tab McManus who was booked for his part in a scuffle.
Orient dominated early on and had enough chances to record their third straight home win as Boston were cut apart in the opening half an hour.
Visiting goalkeeper Nathan Abbey was the star of the show as he pulled off a series of excellent stops.
The game was only seven minutes old when Abbey made his first stunning save - when he dived full length to push Wayne Carlisle's effort around the post.
The visiting keeper then pulled off an instinctive stop from skipper Gary Alexander's close range header following Matthew Lockwood's corner.
As Orient piled on the pressure they forced five corners in the opening fifteen minutes and almost broke the deadlock when good build-up play led to Carlilse crossing from the right but goalkeeper Abbey saved smartly at the feet of Andy Scott.
Boston were forced into a change on 41 minutes when Dean West was stretchered off following a clash of heads with Scott and Mark Greaves was thrown into the action two minutes before half-time.
In the second half Orient went off the boil and the game degenerated into a niggly affair with chances few and far between.
But Orient almost won the game three minutes from time when substitute Jabo Ibehre went on a wriggling run but saw his shot run agonisingly wide of the far post.
That was the last of the chances but there was still time for Orient defender White to be shown the red card by Bristol referee Steve Tanner following an off-the-ball bust up with Boston sub McManus.