This is a SMB3 brick block. Comes with a readme for changing it into: SMB1 brick, SMB1 brick (Demo world style), SMB3 brick, SMB3 brick (DWS), SMB3 coin, SMB3 coin (DWS). v1.3: better way to do the last two.
v1.4: fixed the glitch that a mushroom comes out when small mario hits a block.
Added SA-1 support and screenshots in the process.
The reason why a mushroom could have popped out of the block in the older version is because SMW's routine to spawn a bounce sprite is coupled with SMW's routine to spawn sprites from a block meaning that with the wrong input, a sprite (here a mushroom) may come out of the block when spawning a bounce sprite (or even crash the game).
Regardless of the bug being, I recommend you on using proper brick blocks as the implementation on this block is quite hacky (the block stays visible and doesn't become invisible when you hit them small) due to SMW's limited block spawning routine.