Happened as I went to go play Trickster with osu! still open. I blame the gameguard it uses, but here's the bug report anyhow.

An error has occurred.

System.InvalidOperationException: Begin cannot be called again until End has been successfully called.
at Microsoft.Xna.Framework.Graphics.SpriteBatch.Begin(SpriteBlendMode blendMode, SpriteSortMode sortMode, SaveStateMode stateMode, Matrix transformMatrix)
at Microsoft.Xna.Framework.Graphics.SpriteBatch.Begin(SpriteBlendMode blendMode, SpriteSortMode sortMode, SaveStateMode stateMode)
at osu.Graphics.Sprites.SpriteManager.Draw()
at osu.GameModes.Editor.Draw(GameTime gameTime)
at Microsoft.Xna.Framework.Game.Draw(GameTime gameTime)
at osu.GameBase.Draw(GameTime gameTime)
at Microsoft.Xna.Framework.Game.DrawFrame()
at Microsoft.Xna.Framework.Game.Tick()
at Microsoft.Xna.Framework.Game.HostIdle(Object sender, EventArgs e)
at Microsoft.Xna.Framework.GameHost.OnIdle()
at Microsoft.Xna.Framework.WindowsGameHost.ApplicationIdle(Object sender, EventArgs e)
at System.Windows.Forms.Application.ThreadContext.System.Windows.Forms.UnsafeNativeMethods.IMsoComponent.FDoIdle(Int32 grfidlef)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
at System.Windows.Forms.Form.ShowDialog()
at System.Windows.Forms.Application.ThreadContext.OnThreadException(Exception t)
at System.Windows.Forms.Control.WndProcException(Exception e)
at System.Windows.Forms.Control.ControlNativeWindow.OnThreadException(Exception e)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at CallWindowProcW(IntPtr , HWND__* , UInt32 , UInt32 , Int32 )
at MouseSubClassFunc(HWND__* hWnd, UInt32 msg, UInt32 wParam, Int32 lParam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at Microsoft.Xna.Framework.WindowsGameHost.Run()
at Microsoft.Xna.Framework.Game.Run()
at osu.Program.Main()

Peppy is aware of this I think. It already happened to me thrice already.